Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
TDS is total dissolved solids, and includes fertilizers. That could make it rise. Also, if you are topping off the tank with water that has a high mineral level, but this is usually slower because you are not adding very much water each time. Drops of water can cling to the TDS meter, and evaporate, leaving the minerals behind. The next time you test this could foul the reading. Make sure you dry the prongs of the meter after use.
TDS of around 200 is fine for most community fish, and perhaps a bit on the high end for very particular soft water fish, but most should be OK. Maybe not wild caught, though. Certainly depends on the fish!
GH is a measure of General Hardness, and mostly is calcium and magnesium. These are required minerals for the plants and the fish. This is one of the most important parameters as far as fish go.
KH is carbonates and bicarbonates. These act as a buffer to the pH. Higher KH almost always means the pH will be high and difficult to change. Low KH means that something else can control the pH. The pH may be low or high, and it may fluctuate.
Both GH and KH are measured in several types of units. ALWAYS INCLUDE UNITS.
From your numbers I suspect you are reporting these in ppm. German degrees of hardness is also common.
GH of 60 ppm is quite soft, well suited to most soft water fish. 60 ppm = 3.3 German degrees of hardness.
KH of 120 ppm is high enough to stabilize the pH. If your fish demand lower pH it might be difficult to change. 120 ppm = 6.7 German degrees of hardness.
pH in the mid 7s is fine for most community fish. Sort of at the high end for soft water fish, and at the low end for hard water fish. Did you measure this after the CO2 had been running a while? Or after it had been off?
Nitrate = 10 ppm is a reasonable value. Plenty for the plants, not a problem for the fish.
Drop checker = green means not enough CO2.
Alternate means of testing CO2: Allow all the CO2 to off gas and test the pH. Then run the CO2 and test the pH the next day. If the pH has dropped by 1 this means there is something close to 30 ppm CO2. Usually this means the drop checker will be yellow, perhaps with a tint of green.
How to lower TDS: Do water changes with water that has lower TDS.
Do not do one massive water change that results in a big drop in TDS all in one go. Adjust the water change volume or the TDS of the new water so that the water change results in the TDS dropping by about 10% for delicate fish or a bit more is OK for more durable fish.