Take with shared sump Q

[mermaid_life]

I recently plumbed my small 4 gallon spare tank to my existing display sump. I thought it was going to be plug and play but something weird is happening.

My small tank has a dino problem but my main DT looks fine to the eye and parameters are stable via trident. I thought that since they're both sharing plumbing, there would be no start up phase. Am I wrong? What am I missing?

 

[BaliReefBox]

not knowing but maybe bare new glass, very interested as I was thinking of doing the same for a small frag tank. i might have to rethink...pls keep us posted cheers and good luck