Help

Ok let me start off by saying we are new to this and still learning. Being doing alot of research before we buy anything and also talking with our local fish store owner. We started off very slowly as well. Had the tank with a couple of live rock only in it for about a month/month and a half before we put anything in it. Got a cleaning crew started in the tank and then added fish and coral. Had the tank about 5 months now and in the past month we have lost 5 fish. Some of which we had only had in the tank a few days and some for a month. We do a 2.5/5gal water change every tuesday. Check the levels all the time. Everything is looking great. Coral are growing. But we can't figure out why the fish dying. We did just learn we had voltage in the tank and got something to take care of it but no clue why fish are dying. As of right now we have our cleaning crew, 2 shrimp and 2 clownfish and all the coral. We have a 20gal. Tank, did water change yesterday and levels checked yesterday were nitrite 0.01, nitrate 5, ammonia was 0.0, salinity 1.025,temp.79.3,phosphate was high 2.8-3.0. Thats the highest it's ever been. Normal at 0.3-1.7. Please help why are my fish dying. I have 5 very sad kids.