Just a few questions about tank mates ;D?

You really need to be careful with additions, as plecos can be very messy fish, and I have no idea why they have two large ones in a tank this size.

You can't add koi to anything but a pond or a massive tank, and they are not tropical like all the other fish. you need to be careful adding angels or any other cichlids with the firemouth in there. tetras can work if you stock them properly, if they are larger tetras, as the firemouth will eat small ones.

What I would suggest is that before buying anything you should browse around different dedicated fish websites and forums and learn about aquarium keeping in general, as well as the different types of fish, even read a book on them. Don't rush, and don't trust the pet store employee either. I would suggest that you research yourself any ideas, so you don't get lousy advice from other people who don't know better, that will just cause harm, like koi or "african cichlids"

i would highly recommend selling the plecos to make room for fish that you do want - they really belong in a bigger tank anyway, because if they are common plecos, and fit in your tank, they still should have lots of growing to do.

FYI freshwater aquariums need no salt.

Koi is a no no. Angel fish is a no no as well as A they are Cichlids and can be extremely agressive and B the flat sides of Angel fish are very inviting to a Plec, Tetras will be a nice diet supplement to the firemouth as well ! Also watch out 2 Plecs in the tank will lead to trouble eventually, try to keep fish of the same size and remember the Cichlids are non community, you may have to rethink your whole tank to get the right balance! Good luck :))

Plecos get big and who ever told you about a koi going into a 55 is wrong koi can get up to 10+ and so can plecos. If u want to get some tetras they should be fine, maybe even some African cichlids.