Go Fallow or Not?

[Nolan Cash]

Hey everyone.. so i’m fairly new to the hobby. I have a 40 gallon AIO that is about 1.5 months old. Started with dry rock and used some nitrifying bacteria and 2 clowns to help cycle the tank. Everything did extremely well, water changes every week. Got a juvenile yellow tang about 2 weeks later (got talked into it by my LFS, i know i know i’ve done more research since).

Tanks had 20 hermits, 10 snails, cleaner shrimp, pistol/goby pair, 2 clowns, yellow tang

Parameters (api testing kit)

salinity 1.026
temp 77
ammonia, nitrate, nitrite 0
ph 7.4

tank was great when I left for a week to go council a church camp. came back to some tiny white spots on yellow tang, i assumed it was ich but i didn’t have a QT tank at the time. So i watched the fish like a hawk.. spots went away after 2 days and everyone was healthy and eating.

decided to do some maintenance by stirring up sand bed to move waste and diatoms. a water change hadn’t taken place in 2.5 weeks bc of the vacation, so that was going to be done the next day.

i wake up the next day to some pretty lethargic fish but i didn’t think anything of it since lights were still off. came home 8hrs later to a dead yellow tang, was hard to see if he had any white spots as he had sand on him too from being in the sand bed. clowns were struggling.. just laying in sand bed gasping for air.

I had setup at QT the day before so i moved the clowns, goby and his shrimp friend (just to keep them together) into the QT. Clown have almost fully recovered, they are eating and swimming about 75% of what they used to.

All this to say, what went wrong? Should i go FALLOW? How to treat QT tank?

 

[Tcook]

Tank seems to young for a deep anoxic zone in the sand bed and stirring up H2S. I think you may have stirred up a lot of nutrients leading to a bacterial bloom and dropping your O2 levels. How much surface agitation do you have?