90 l tank with plants and 17 fish. 9 neons 3 tiger barbs 3 glowlight and 2 cherry barbs. previous tests were 0 on ammonia 0 nitrite 40, nitrate 40 and ph 8.4. i did a 50 %water change on tues as part of my ususal maintenance then tested 1 hout after and got 0.25 an ammonia ph 8.2 nitrite 0 and...