What else should I be testing?

[MarkoD]

i got ich in my tank about 3 weeks ago and lost 5 out of 8 fish.

since then i have been trying to figure out why this happen and i still cant.

i've been testing my water religiously, i even bought a new test kit thinking my previous one was old and inaccurate.

for the past 2 weeks I've been testing and i've been getting a constant reading of:

ammonia: 0
nitrite: 0
nitrate: below 5 but barely detectable
PH: 8.2
temp: between 25.9(night) and 26.5(day)
specific gravity: 1.023

is there anything else i should be testing that could be stressing out the fish?