Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
TDS measures all the stuff in the water (especially charged particles, but that is almost everything) It is not a specific measure for harness (either General Harness or Carbonate Hardness). As a very rough guide TDS under 200 ppm is pretty soft, 200-800 is middle range to somewhat hard, and over 800 is quite hard. If your TDS says under 200 or over 800 I would want to dig into the exact values for GH and KH.
If you can get a water quality report from your water company that might have all the info you need. General hardness may also be called total hardness, and carbonate hardness may also be called alkalinity. Either may be measured any of several ways, one way is to report the value as ppm-CaCO3 meaning parts per million as if it was all from calcium carbonate (it usually is, but not always)
The other thing to look for in your water quality report is separate tests for calcium (Ca) and magnesium (Mg). These are highly likely in ppm.
You can go to the nearest fish store (not pet store) and ask if they are on the same water system as you. If they treat the water at all. What the test results are for that water.
You can take a sample of your water to a store that tests it for you. Some have a small charge. Get the actual numbers, the test results, not just a comment like 'It is OK'.
An informal water hardness test: Do you have white or off-white crusty stuff around sinks? If yes, these are almost always the minerals that indicate hard water.
Another informal test: Does soap lather well? Or not? Soap does not lather well in hard water.
Another informal test: Do all your neighbors have water softeners? It is probably because of hard water, though 'keeping up with the Jones' factor may play a part.
Now... do you need to do any of this?
You might not need to. I would look at the water quality report and go from there. If any of the informal tests I suggested say you have extremely hard water or extremely soft water you might want to look into it more carefully.
Most aquarium plants are quite adaptable to a wide range of water harness. There are only a few that really require soft, acidic water.
Most fish that have been hatchery raised are tolerant of a wide range of water hardness. There are many fish, however that still require either very soft or very hard water. If you know what your water is like then you can select fish that are best suited to that water.
If you decide you want to keep fish that are not suited to your water then you will need GH and KH tests to help you set up and maintain water for those fish.
If you are willing to work with the fish that are suited to your water then all you need to do is find out what that water is once. While water company water can change over the year, depending on the source, it usually does not change that much. You can ask the water company about that, too.