Do fish deplete alk

[Lee belk]

Do fish deplete alk cal and mag
I’m half way through qt on my friends fish
We put them in qt when we realised he had ick we left main tank with corals in it we have been testing his main tank every week to keep it in balance but his tank is not using nearly as much as it was with fish in it the tank has been fallow for 8 weeks

 

[Jonify]

Fish don't, but the waste they produce does. The nitrogen processing bacteria consume alkalinity to complete that process. So if you have less fish, or if you're medicating a tank and the medication is impacting the bacteria, alkalinity consumption would theoretically go down.

 

[arking_mark]

Fish don't, but the waste they produce does. The nitrogen processing bacteria consume alkalinity to complete that process. So if you have less fish, or if you're medicating a tank and the medication is impacting the bacteria, alkalinity consumption would theoretically go down.

My understanding is the production of nitrate consumes Alk. However, the other half of the cycle that reduces nitrates returns the exact amount consumed. So there is no net change in a cycled tank unless over time, nitrates are rising or falling.

My bet is without the fish the coral is getting less nutrients and therefore slowing Alk/Ca consumption.

 

[Randy Holmes-Farley Verified Community Expert]

If nitrate is rising from fish waste then it is depleting alk, but never calcium.

If nitrate is steady between water changes, regardless of the value, then the nitrogen cycle overall (including fish waste) is not depleting alkalinity.