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What is TDS

AQUARIUMS & AQUACULTURE: Water and the health of your fish or reef

  • The ability of water to dissolve, combine with, or suspend other elements and compounds can be helpful as a supply of necessary nutrients. Water can also be a source of trouble for the aquarist because it can contain chemicals that are irritants or toxic to aquatic organisms. Aside from overall stress, water quality is the most common cause of fish diseases. 1
  • A constant level of minerals in the water is necessary for aquatic life. Changes in the amounts of dissolved solids can be harmful because the density of total dissolved solids determines the flow of water in and out of an organism's cells. Concentrations that are too high or too low may limit the growth and may lead to the death of many fish or reefs. 2
  • A level of 400ppm or less is recommended for most freshwater fish although many softwater fish demand a significantly lower level. Conversely, saltwater fish require a very high level of anywhere from 5000 to 50,000 ppm. Total Dissolved Solids are also important for proper osmotic regulation, which is the relationship of water versus dissolved solids in the cells and the external environment. The greater the amount of solids in the water versus the solids in the tissue of the fish will result in a fluid loss via the gills. 3
  • Authorities indicate that water of low ppm and pH control may be more desirable for breeding, though this depends on the species of fish. Since total dissolved solids content of a softened water is the same as that of the untreated raw water, a supply with a lower dissolved solids content must be gained in some other way. Blending of softened water with reverse osmosis or distilled water may produce the conditions conducive to aquatic breeding. 4
  • It is critical to regulate the alkalinity & buffering capacity (pH) of the water as well as the amount of Dissolved Oxygen (DO) present in your aquarium, tank or pond.

It's essential to know your fish and your water.

HM Digital's TDS-4TM and DM-1 are perfect for aquarium owners and aquaculture. It's the ideal tool to measure the effectiveness of a water filter to ensure the water simulates a natural habitat as closely as possible, whether the water is for salt water or fresh water fish tanks, ponds or reef tanks.

The DM-1 can be used with a reverse osmosis system or RO and distillation or other system combination to ensure tank water is pure prior to being treated. It is the best device available to gauge if your purification system is effective. Since it has two lines, the unit can work measuring tap (feed) water and purified water or by measuring the product water of a two-stage system (e.g., RO & DI).


1. Fairfield, Terry. A Commonsense Guide to Fish Health. Barrons. 2000.
2. Lehigh Envirosci.
3. Fairfield
4. WQA : Is soft water safe for tropical fish?

Additional Sources:

  • Water Quality Considerations for Aquaculture


Nutrient Levels

Hydroponic gardeners all over the world monitor nutrient levels in roughly the same way-by measuring total dissolved solids in the growing solution - but they have different preferences for the format of their test results. Growers in the U.S. often examine a simple ratio known as parts per million PPM). High concentrations of salts increase the solution's electrical conductivity (EC).

Total dissolved solids readings help to indicate the density of solution which can help to determine when it's time to add nutrients. Temperature can affect TDS/EC meter readings. It is important that you read your hydroponics nutrient packaging carefully to make sure feeding indications correspond with your meter.

Information below provided by Sunleaves .

Suggested PPM Levels

For great results, consult the ppm table below for assorted fruits, veggies, herbs and tropicals.
Keeping your nutrient solution at the right concentration is important for maintaining optimum growth!

Species TDS Range         Species TDS Range
African Violets 840-1050         Ficus 1120-1680
Artichoke 560-1260         Garlic 980-1260
Asparagus 980-1260         Gladiolus 1400-1680
Aster 1260-1680         Impatiens 1260-1400
Bananas 1260-1540         Lavender 700-980
Basil 700-1120         Leek 980-1260
Beans 1400-2800         Lettuce 560-840
Blueberries 1260-1400         Mint 1400-1680
Broccoli 1960-2450         Okra 1400-1680
Cabbage 1750-2100         Palms 1120-1400
Carnation 1260-2450         Parsley 560-1260
Carrots 1120-1400         Peas 980-1260
Cauliflower 1050-1400         Rosemary 700-1120
Celery 1260-1680         Sage 700-1120
Chives 1260-1540         Spinach 1260-1610
Cucumber 1190-1750         Strawberries 1260-1540
Eggplant 1750-2450         Thyme 560-1120
Endive 1400-1680         Tomatoes 1400-3500
Ferns 1120-1400         Zucchini 1260-1680



"TDS: Check it out when the water doesn't behave."
By David Dickman
Reprinted from Pool & Spa Service Industry News

TDS - Total Dissolved Solids - may be the most misunderstood factor in the whole field of pool & spa water chemistry. It is misunderstood because no one knows exactly what effect it is going to have on any particular body of water.

When everything else seems to be all right, and the water still acts screwy, check the TDS.

High TDS can result in corrosion of metal equipment and accessories, even though the water is balanced.

High TDS can cause eye and skin irritation, even though the pH is right and there are no chloramines in the water.

High TDS can permit an algae bloom, even with a 2-3 ppm chlorine residual.

But none of these symptoms will necessarily occur. It is the uncertain nature of problems caused by TDS that makes it such a headache to deal with.

As its name states, TDS is the sum total of all of the dissolved things in a given body of water. It's everything in the water that's not actually water. It includes hardness, alkalinity, cyanuric acid, chlorides, bromides, sulfates, silicates, and all manner of organic compounds.

Every time you add anything to the water, you are increasing its TDS. This includes not only sanitizing and pH adjusting chemicals, but also conditioner, algicides, and tile and surface cleaners. It includes airborne pollutants and bather waste as well as dissolved minerals in the fill water.

At low levels, TDS does not present a problem. In fact, a certain amount of TDS is necessary for water balance. Hardness and Total Alkalinity are both part of TDS.

But at high levels - above 3,000 parts per million - you are welcoming problems. The National Spa & Pool Institute, in its standards for both swimming pools and spas, recommends an ideal TDS of between 1,000 and 2,000 ppm, with a maximum of 3,000 ppm.

No one knows exactly what mechanism is at work when you're dealing with high TDS water. One commonly held theory is that when you get a lot of dissolved substances in water, they interfere with the normal workings of sanitizers. They may do this by forming a chemical "shield" around bacteria, algae, and other substances normally attacked by chlorine or bromine. Or they may simply present "roadblocks" in the path of sanitizer molecules, preventing them from freely circulating.

TDS buildup is inevitable. As we've explained, every time you add chemicals to water, you're increasing the TDS. When the water evaporates, it leaves behind all of the solids that had been dissolved in it.

Just how much are you increasing the TDS when you chemically treat pool or spa water? Well, for every pound of dry chemical that you add to a 15,000 gallon pool, you are increasing the TDS by about 8 ppm.

"Parts Per Million" is what scientists refer to as a "weight to weight" measurement. That is, if you know how much the water weighs, and you know how much the stuff you are adding to the water weighs, you can calculate how many parts per million you are adding to the water.

As it happens, one gallon of water weighs 8.34 pounds. So if you know how many gallons are in a pool or spa, and you multiply that number by 8.34, you will know how many pounds of water are in the pool or spa.

So, how many pounds of water are in a 450 gallon spa? The answer is 3,753 pounds (450 x 8.34 = 3,753). How about a 15,000 gallon pool? Well, 15,000 x 8.34 = 125,100. So, there are 125,100 pounds of water in a 15,000 gallon pool.

OK, so now you know how much the water weighs. Now, take 1 million and divide it by the total weight of that water, and you will find out how many parts per million are contained in each pound of water. For example, 1,000,000/125,100 (pounds of water in our 15,000 gallon pool) = 7.99. We'll call it 8. That is, every pound of material added to a 15,000 gallon pool will contribute about 8 parts per million.

How about our 450 gallon spa? Take 1,000,000, divide it by 3,753 (the weight of the water in the spa), and you get 266.45. We'll settle on 266. So for every pound of stuff that you add to a 450 gallon spa, you will be increasing the TDS by 266 parts per million.

Effect of Dissolved Material on TDS - Pools

(How Much 1 Pound of Material Will Raise TDS In Pools of Various Sizes)

Pool Capacity
TDS per 1 Pound

5,000 gal. 24 ppm
10,000 gal. 12 ppm
15,000 gal. 8 ppm
20,000 gal 6 ppm
25,000 gal 4.8 ppm
50,000 gal 2.4 ppm
100,000 gal 1.2 ppm
150,000 gal 0.8 ppm
Effect of Dissolved Material on TDS - Spas

(How Much 1 Pound of Material Will Raise TDS In Pools of Various Sizes)

Pool Capacity
TDS per 1 Pound

400 gal. 300 ppm
450 gal. 266 ppm
500 gal. 240 ppm
550 gal 218 ppm
600 gal 200 ppm
650 gal 184 ppm
700 gal 171 ppm
750 gal 160 ppm

The above charts should give you a pretty good idea of why TDS builds up so rapidly in spa water and also why the effect of chemical treatment is so dramatic when you are dealing with a spa. The accompanying charts - one for pools and the other for spas - will also give you an idea of how much of an impact 1 pound of chemical added to the water will have on TDS in vessels of different sizes.

Incidentally, if you wanted to design a pool that would contain almost exactly 1 million pounds of water, it would be a 120,000 gallon vessel. In that pool, every 1 pound of solids dissolved in the water would increase the TDS by 1 ppm.

Every sanitizing chemical, and every pH adjusting chemical used in the pool and spa industry will eventually contribute to the TDS in a pool or spa. Some will contribute more than others. Because sanitizing compounds often require the additonal use of pH adjusting chemicals, the chemical maintenance regimen you choose can have a dramatic effect on the buildup of TDS.

TDS is fairly easy to calculate for dry chemicals. It's a bit more complicated for liquid solutions. If a research lab were going to test the solids content of a liquid, they would take a precise volume of the liquid and then slowly heat it until the liquid itself had evaporated. Then they would dry the remaining solids and weight them.

The two most common liquid solutions used in our industry are muriatic acid and liquid chlorine (sodium hypochlorite). For your information, 1 gallon of muriatic acid will contribute 1.87 pounds of dissolved solids to the water. 1 gallon of liquid chlorine will contribute 2.2 pounds of dissolved solids.

You should also understand that every type of chlorine & sanitizer - including gas chlorine - eventually ends up contributing to TDS in the form of chloride. So every time you add a pound of dry chlorine compound, or infuse a pound of gas chlorine into a 15,000 gallon pool, you will wind up increasing the TDS by about 8 ppm.

TDS Testing

Short of a detailed laboratory analysis, the most dependable method of TDS testing is through the use of a portable TDS meter. These meters actually measure the conductivity of the water - its ability to conduct an electrical charge, which happens to increase as the TDS increases.

Hand-held TDS meters - usually in the $50.00 - $150.00 range, gggeenerally operate by either placing some water in a sample cell or dipping the meter directly in the pool water and pushing a button, which causes a small electric current to pass between two electrodes immersed in the water and separated by a specific distance.

The meter measures the current passing between the electrodes and uses that to deter mine the water's conductivity, the meter dial (or LCD readout) is calibrated to indicate TDS in parts per million.

The easieste way to reduce TDS is to drain the pool and refill it with fresh water. This can also be done in stages, taking the water level down 1 or 2 feet at a time and refilling over a period of days or weeks.

Under normal circumstances, pool water can be expected to last anywhere from 3 to 5 years before it has to be completely changed. Your choice of chemical treatment can help to determine just how long it lasts.


If you have a filter or RO system in your home, you need to check the water it produces periodically to make sure it's working properly. The performance of RO systems and filters are measured by the amount of TDS Reduced by the filters and membranes. The reduction of TDS indicates the reduction of microorganisms and harmful non-solids such as chlorine and fluorine.

So be sure to check the quality of your water every month to make sure your filters or membranes are working well!

Carbon Filters

Granular activated carbon filtration is the most common technology used in home filter systems. Unfortunately, these home systems are often poorly maintained. In many cases, filters are not cleaned properly, or filter elements are not changed at appropriate intervals. Over time, effectiveness declines, and in some cases the contaminants in the overloaded filter actually begin to discharge back into the water.
RO Percentage Rejection

The effectiveness of an RO unit is characterized by the rejection rate or rejection percentage.
The rejection rate is the percent of a contaminant that does not move through, or is rejected by the membrane. Rejection rates need to be high enough to reduce the contaminant level in the untreated water to a safe level.
To determine the needed rejection rate, it is necessary to consider the initial concentration. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively).
An RO system design is based on a certain range of feed water TDS, the percentage of rejection and percentage of recovery desired. For a given system, the higher the percentage of recovery or the lower the percentage of rejection, the poorer the quality of product water becomes. (US FDA)