[CrownMan]

Cardinals will do fine in soft water. Shrimp will need calcium though for their molting cycles. The arlington water has about 20 ppm Ca with the KH about 4 degrees out of the tap. If you are doing your water changes regularly with tap, then you should be just fine. I have 7 tanks now with Cherry and Yellow shrimps (seperate tanks of course as they will interbreed). Good choice choosing cardinals. Mine are about 6 years old now and quite large (about 2") and they used to school well when I had more of them.

 

[CrownMan]

I use Equilibrium in all of my tanks but if you follow the instructions to add 1 degree of GH, it will add a lot of potassium to your system. That works in some of my tanks that I don't use EI and CO2. It is a powder and you can use the fertilator in this web site to play around with different doses of it to determine how much Ca, Mg, Fe and K it adds. The instructions on the bottle state that to increase GH by 3 degrees, add 3 tsp per 20Gallons. That's 41ppm of Potassium you are also adding. You might want to buy a tub of pure CaCl from Leslies Pool Supply and a bag of Epson salts (Mg) to do the same thing without adding so much Fe and K. I only have used it in 1 tank to raise my GH and that is my 29 Gallon tank that occassionally bottoms out the Gh and Kh due to other factors. That 29G tank has a seasoned substrate that keeps the pH around 5.9. It houses about 100 cherry shrimp, 3 Peppered Corys and 6 Albino Corys. No CO2 in that tank.

BTW, when I change my water in my Amazonia tanks, I get even larger pH swings. It hasn't harmed one fish or shrimp. In fact, my male shrimp become very active afterwards. My pressurize CO2 system with controller kicks in and brings it back down in 30minutes. I have Cherry Shrimp, 10 Amano Shrimp, 15 otos, 10 Nerites and 10 Pygmy Corys in that 40G tank.

I agree with Ed on his observations about the effects of pH and TDS on fish and shrimps.