eddieperth

Hello all!  Longtime reader, first time poster

I have a 4x2x2 lo-tech planted aquarium.  Aquarium has been set up for a year now.  Fish and plants generally thriving and growing. Water parameters stay at or near perfection, though the tank tends to be acidic. (Typically about 6.4pH)

The Problem:

The last two times I've done a major water change (approx. 30-40%), the tank water has gone cloudy, and within 24hours the fish are gasping at the top of the tank. Both times when I have tested the water, the Nitrite reading was high. (testing at other times shows it at/near 0)

I am mystified as to what might be going on and would welcome any input/ideas as to how to address this!

My only idea is that perhaps something in the tap water is killing off some of the beneficial bacteria???

Additional info that might help:

I live in Kingsley which is NOR.

I do not disturb the substrate when I change water.

I use water directly from the tap without letting it stand (no idea if city of Joondalup uses Chlorine or Chloromine), which I then treat with water conditioner in the bucket before pouring in tank.

Filtration is cannister filter filled with bio balls and a sponge.  I do not run an airstone.

Tank is not overstocked- 5 rainbows, 2 juvie angelfish, 5 corys, 2 juvie plecos, 3 platies, 2 kribs

If more info or pics are needed, let me know!  Thanks in advance for any help!