Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
GH is a measure of calcium and magnesium. This is the hardness that you want to test when you want soft water fish or hard water fish.
KH is a measure of carbonates. Carbonates are one of the main buffers that stabilizes the pH. High KH means the pH will be very difficult to change. Low KH means the pH will be easier to change.
pH is a ratio of OH-: H+ in the water. Most fish are not so particular about the pH, but your pH way over 8 is pretty extreme. pH also controls how certain fertilizers are available to the plants. Optimum pH for fertilizer availability is pretty close to neutral.
Peat moss does not always react the same. Some peat moss is almost as active as a sodium exchange water softener, removing several of the minerals. Other peat moss simply adds some organic acids, and does not do much with the minerals in the water. No way to tell without actually testing it. I have seen posts from some people who did some careful tests and saw that the GH and KH of the water dropped significantly. In my case the peat I have does not seem to do much in that way. Sure helps lower the pH, though!
TDS is a measure of all the things in the water. Pure water does not conduct electricity very well. The more stuff in the water (especially charged particles like dissolved salts and minerals) the better it conducts electricity. A TDS meter can tell you how much stuff is in the water. The stuff may be minerals, salts or other things. Soft water fish want less minerals, salts and other things in the water. Hard water fish want more minerals in the water. A TDS meter does not tell you what the things are in the water.
Here is how I would handle your situation:
Get a gallon of RO or distilled water (they are about the same as far as aquarium use goes).
Make several recipes blending tap with RO and test.
25% RO + 75% tap
75% RO + 25% tap.
Test GH, KH, pH and, if you have a meter, TDS.
If none of these are right, mix other blends until you get pretty close.
Then start with whichever one is the closest to what you want for GH and KH (for Cardinals I would aim for GH and KH about 3 German degrees of hardness, or about 60 ppm)
What is the pH? For Cards, I would want the pH pretty much anywhere in the 6s, though lower 6s is best. pH is not as critical as GH.
If the water still has a pH above neutral I would add some peat moss.
Try a few cups of water and 1/4 cup of peat moss. Stir it several times and test the next day. If you have a small pump (air or water) this can stir it for you.
Test GH, KH, TDS, pH.
IF the peat moss you have removes the GH and KH really well, you might be able to start with more tap water in your mix and rely on the peat moss to remove some of the minerals.
IF the peat moss just drops the pH, that is OK. It will likely turn the water a little bit brown or yellow.
Now that you have something of a recipe...
X amount of tap water
X amount of RO or Distilled
Circulate it with peat moss overnight
...here is how to use it:
Fill the tank with the recipe.
Every time you need to do a water change prepare the water the day ahead (peat moss takes a day or so to work).
When you buy new fish:
1) Set up a quarantine tank that matches the main tank.
2) ASK what they are in: GH, KH, pH, TDS. Do not always trust the answer.
3) TEST the water in the bag when you get home.
4) Add minerals to the quarantine tank as needed (in your case add tap water) until the Q-tank matches the bag water. (Suggestion: If the store is on the same water as you, you might just fill the Q-tank with tap water)
5) Keep the new fish in quarantine for a month, watching for and treating diseases or parasites. Do water changes to gradually alter the water to match the main tank. For example, twice a week do 25% water changes using the recipe to refill the tank.
When the fish are clear of diseases and parasites, and are acclimated to the new water, move them to the main tank.