I have 46g bow front with Malawi for about 7 months. I have about 13 fish in sizes from 1in to 4in. I have plenty of filtration: Eheim 2213 as bio only and two AquaClear 300 with sponges and crushed corals for pH buffering. I change about 25-30% water every 7-10 days to keep my Nitrate around 20-40 ppm. I condition water for 24 hours in a 20g garbage pale with correct temp. Then I add 1 tsp of each" Cichlid chemistry, baking soda and epsom salt to about 15g of water in that pale. I vacuum clean the gravel only once a month, the rest of the time - just drain water and pump from the pale over 20 minutes. Something happen yesterday. After I added new water, I noticed fish was staying at the top. This morning demasoni, yellow lab and pseudo are dead and 2 more smaller fish are laying down - ready to go. The larger fish came to eat this morning but the smaller once did not even came from the rocks. I checked the water this morning: Nitrite - 0, Nitrate - 20, pH - 8.0. I do the same routine every week. Fish always healthy and have a nice color. The only several deaths I had before is when the new fish was attacked by old ones - usually within the first 3-4 days. What could go wrong? I am puzzled.