We have a digital temp gage that we go by before adding the new water to the tank so it is the correct temperature. We had to do the 50% water change in two parts since we only have a 20 gallon trash can to use. Our aquarium holds 84 gallons, but with the gravel and fake plants, I figure it comes out to more like 75 gallons. The temperature in the first batch was slightly lower then 85 degrees, the next batch was slightly higher then 85 degrees. I was sure to add the correct amount of water conditioner after I had filled up the trash can half full on both batches. I also added Aquari-Sol, 21 drops for each 20 gallons of water. The instructions say to add 12 drops for each 10 gallons of water. I also added 4 Tbs. of aquarium salt to each batch. We have been doing 25% water changes every week, ammonia 0, nitrite 0, and as of today nitrate was 40, that's why we did the 50% water change. PH is 7.6, we get our water from the Colorado river and the PH is the same for all the fish stores. I tested the water again about an hour ago and the readings are ammonia 0, nitrite 0 and nitrate 10. We have had the red tailed shark only about a month. We had added two new bristlenose catfish January 2 which we had purchased from our local aquarium shop (not Petco or Petsmart) Before 24 hrs. was up, one of the bristlenose died and the next day we noticed the ick on our pictus catfish. Yesterday the other bristlenose died. Our aquarium shop is the one that recommended the salt and Aquari-Sol and where we also purchased the bristlenose from. Before this we had not had salt in our tank. This is so depressing to lose fish so quickly. The pictus catfish are thin but hanging in there. Do you know what we did wrong?