Cichlids in new tank

[Dane.w]

I set up a 40 gallon for my two firemouth cichlids as they were aggressive to other fish, I put some old and new filter media in the tank and they have been doing okay apart from staying in the bottom half of the tank most times and sometimes swimming at a 45 degree angle so not fully on the side but still at an angle, I tried to get pictures but the reflection is quite strong and I don’t have a light yet (I have one just need to set it up), is this something to worry about or do they just need time adjusting

 

[Colin_T]

its because there is no substrate and no backing on the tank. the fish are stressed out.

 

[Dane.w]

I’ve added some sand what could I do for backing because that black back is where the water goes in and out for the filter

 

[Colin_T]

if the tank has black plastic on the back for the filter, that is fine.
add some plants and a cave and they should feel better.