400 gallons fish recommendation.

[Fanatic_Fish_Lunatic]

Smaller cichlids will be difficult to view from the top.
Maybe a big catfish or 2; red tail cat, tiger shovelnose, something you can see easily
Some nice koi, though not predators, would look great as well

His dad already have a separate pond full of koi.

 

[Nicefishtank]

What about an African cichlid mixed mbuna tank for color?

 

[Fanatic_Fish_Lunatic]

What about an African cichlid mixed mbuna tank for color?

I actually suggested him that. I just need to convince him more, lol.