Why TDS is the secret to a thriving fish tank
- Jan 26, 2025
- Anshika Mishra
- 23 0 0
TDS stands for total dissolved solids. It's usually tested with a simple TDS meter. TDS is just a fancy way of saying that here's a number representing everything that has dissolved into your water column. The big one that we'll use for our benefit is Nitrate.
The Chemical Fear
Chemicals like Prime or Safe and Exvel or Buffer are usually supposed to be bad things. However, the source water can already contain a lot of these chemicals. However, water (H2O) is also a chemical.
While TDS can tell you what is dissolved in your water, it cannot tell you how much of each substance is present. If your source water starts at 100 parts per million, you won't know what is in those million parts.
Let's say you have 50 big fish in your tank that produce a lot of waste. You feed about twice a day, and during your water changes, you add prime, salt, buffer, and Cichlid trace. Through this, you might end up with 500 parts per million of TDS.
You get that reading because of all the chemicals you've added to the African Cichlid tank. You must adjust your source water parameters to match the water your fish thrive on. This could mean raising the pH and hardness, thus adding chemicals.
If 400 of those 500 were copper in your water, it would be bad. However, you must utilize the information a TDS reading gives you correctly. The number is meaningless on its own. But you can use it to measure the changes in TDS over time.
How to Use the Information from TDS?
As mentioned, TDS includes the nitrates that build up in your tank in between water changes. So, this is how you can use that information to your benefit. As experienced hobbyists, we do most of our water changes and tank maintenance on a scheduled basis. Our tanks are mature and established. We did all the testing a while ago and know that our tank needs weekly or monthly water changes.
For beginners or new tank setups, you should test your water continuously to ensure that it has cycled and to see how fast your nitrates rise in the tank. This is how we know when it may be time for a water change. Whatever the trigger level is for the water change, be regular about it.
The most common in the hobby is 40 parts per million. Some people let it go higher before the water changes, which works for them.
But here's a quick hack for using TDS to determine when it's time for a water change: Start by testing the TDS in your tank when it's freshly cleaned. Then, add whatever chemicals you regularly add. This will be your foundation.
How to Use TDS?
If you do weekly water changes, you should manage the tank as you normally would throughout the week. At the end of the week, when it is time for the water change, do your Nitrate test as you usually would and confirm that you reached the 40 or 60 level that triggered the change. Now, test your TDS again.
Your TDS will be higher than you started at the beginning of the week. Of course, this is due to the fish food and waste. But it also includes the nitrates that were rising throughout the week. This water change day TDS you just got is what you can now use to trigger your water changes.
You can also use the difference between the end and the beginning of the week to determine how much TDS rises in that weekly time frame. Start with weekly 200 and end with 300. This way, you'll know that your regimen and fish are adding about 100 TDS every week.
If you determine how much TDS rises in any standard week, you'll know what TDS to expect at the end of the week, which would be your water change trigger.
However, if the TDS rises abruptly some week, you may want to search for a dead fish or a dead plant or trap particles of food somewhere. Keeping track of your TDS is as easy as it is to stick that meter into the water. It is a great way to quickly tell you that your tank is on schedule and nothing unusual has happened.
This does not mean that you can never change anything or try a new product. It just means that you should take a new base-level measurement at the beginning of each week and expect your end-of-the-week measurement to adjust accordingly.
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