180G tank stock

[Lazybum34]

I currently have,

1x endlicheri bichir 11-13"
1x senegal bichir 6"
1x pseudopimelodus bufonius 6"
1x bgk 6"
1x tiger oscar 13"

In my 180 gallon and I was wondering if i am at max stock? If not I would like to know some mid water column swimmers.

Thanks in advance.