I have a 40b guppy tank, started 10/16, no new fish since early November. 2 ac 70s, sunsun 304b, plus a sponge (sponge is for hospital if I needed it, have never took it out to use). Heavily planted. Six oto cats, 1 bristlenose, numerous guppies, rcs, nerites. This am I found 8 dead guppies. Tested water ph 7.6, kH 6, ammonia nitrites 0, nitrates 30. Did not dose ferts or excel yesterday (meant to wc yesterday but didn't get to it) or today. No new fish. No new decor. No new plants. Removed some plants last week. Topped off two gallons water Monday am. Do 50% change every week, use prime. 8 found this am at lights on, tested water, dosed prime x2 for volume of tank. Came back after taking kids to school to 7 more dead. Immediately did 65% change, more dying, current count is 27. Dosed tank w neutral regulator (also dechlorinates) during wc in case prime wasn't working..., pulled some out and placed in bucket w/ totally new prime bottle, not helping... Please help!!!!!!!