But, I have my 40 breeder. Hmmm.
It was stocked with a RTBS and 8 scissortail rasboras. The shark was a small juvenile. I lost him last week

Not sure why as he had been fine. But about 6 weeks ago, I didn’t realize my python connection had loosened and I introduced a ton of tiny bubbles into the tank. I lost one Rasbora and the shark hid for 3 days than seemed ok. But maybe he had lasting damage?
Anyway, I know if I get another shark, he will eventually need a longer tank, correct?
So then I started thinking about stocking it with the sids.
So is this a better plan? Tell me about sids.
And the main question is do they do better on a sand substrate? My 40 is gravel so I would need to change out the gravel. Or would a mix of gravel and sand work?
Thanks all!
And yes, for those on Fishlore, this is a duplicate post!