hmmmmmmm, ok, uh, i really don't know what to do tbh then, I really messed up but I seem to have been doing fine, my loaches are quite active too, my fish all seem healthy, nothing has ich or parasites and they all eat and socialize perfectly fine, I guess I'll have to get another test kit to test it again, my shark is around the 6 inch mark too and he is doing fine, my loaches are maybe 4 inches and are the happiest fish I've ever had. would there be any signs that they are sick/getting sick because you said that my fish are probably just slowly getting sickYou don't want celestial pearl danios unless you're doing a tank for nano fish because the bigger fish will make them hide all the time and you won't see them. Back when I had a krib with mine I never saw the celestial pearl danios. Once I moved him out I see him all the time.
Also I wouldn't do 40 tetras in a standard 29.
Also maybe the shark and dojo loaches shouldn't be in the cube. I have a 6 inch rainbow shark and those loaches can hit 8 inches in time.
The cherry barbs are a schooling fish.
Kuhli loaches are an extremely active fish and in my experience I'd say even a 4 foot tank isn't big enough for them let along a 30 inch tank. Just my personal opinion though.