Need much needed advice on water parameters
I regularly read posts and view photos at this forum,however have never posted. Please forgive the length of this post, butI really need some advice, I have kept a 55 gallon planted tank going for 3 years now. I have never had any major problems with it interms of fish dying or anything like that. Only some algae blooms initially but the tank stabilized over time. I used to do direct tap water changes to the tank evry two weeks and added conditioner directly. Now all my problems of late have started after I bought a 90 gallon planted tank from someone who had discus for sale also. So always having the dream of owning a discus planted tank ,I got the tank and set it up and everything seemed fine for a while, but a couple of times after water changes, the fish have gasped for breath the next day or a couple of days after, so took the water to LFS as well as checked it myself. The reasons I thought at the time was low O2 or chlorine toxicity. The 90 gallon has slow growing plants like anubias and swords. I increased the aeration by adding a couple of more air pumps. The second time it happened I lost three of my discus and all my shrimp! then I thought OK, ph of tap water was high 7.8 compared to my tank 7.0. So i kept water in a spare 30 gallon tank with a heater and the dechlorinating solution in it for a whole week, checked pH, ammonia, nitrites and then changed the water (partial water change ) in my 90 gallon. Everything was fine for the first day,second and I thought Ok problem solved but on the 4th day after the water change,again fish gasping ,most of my shrimp dead! Checked the water, ph was 6.5 and water had nitrites 5-6 ppm! Now am at my wits end, I have no clue what to do!Oh and the 90 gallon has been battling really bad hair algae for a while (its been 3 months since I got this tank). I could really use some advice from someone. i am close to giving up and getting rid of everything! I really don't want to harm my beautiful discus! I would rather they lived healthy in someone else's tank!
PS sorry for the long post