I have been experiencing a consistent decrease in population of Cherry Shrimp in my tank.

I have a nano-tank about 20cm wide. I run a hang-on type filter together with Seachem denitrite placed inside it.

Tank parameters are as follow:

Temp range: 25C (office environment)
Lighting: Recently, 3 - 4 hours. Previously up to 8 hours a day.

Current readings:
NO3: 40mg/L
NO2: 0 mg/L
pH: 6
KH: 0 mg/L
GH: 120 mg/L

I have been using a mix of 3/4 distilled water + 1/4 tapwater (Treated with Prime) to do about 1/3 (or lesser) water change.

I have 2 Golden Algae Eater in the tank as I am experiencing algae growth on the glass.

Any idea what have I done wrong here? Please advise!

Thanks!