Well first thing, your tank is waaaayyyy too small for two Parrots regardless of their sizes. For two Parrots you need at least a 200 liter tank or more. Parrots are cichlids and cichlids are very territorial, they need tons of space to spread out and pick out their own territories. Because you only have one decoration the bigger parrot claimed the tank as his own and will defend his territory at all costs. It's also recommended to have one male and one female even though they can't actually produce offspring.
EDIT: so basically your big fish is picking on your little fish and making him sick. Either get a 200 liter tank or larger or give the fish away/sell them and get fish appropriately sized for your tank.
Anything relatively small, most Tetras, Rasboras, Danios, and cory cats. You'd have to get more decoration though as smaller fish prefer more hiding places so they feel comfortable. It's funny because the more hiding places you provide the more they are out in the open because they feel comfortable!
For the Parrots or different fish? It all depends on your fish, if you have a nice big school of Tetras and a school of Cory catfish you could feed once a day just fine. A pinch of flakes and a high quality veggie wafer for the cory cats. You can even feed them actual veggies like raw cucumber. Some people suggest to blanch the cucumber but my cories/otos/plecos always loved it better raw. Just cut up a slice, weight it down with something like a rock or a fork and let them at it! The first time you do that, they might not find it right away but if you keep putting it in the same spot each time you feed, they'll know where to find dinner ^_^
Plants, I definitely suggest plants! Live plants is a very different story and if you want to tackle them, you'll need to do plenty of research of plants and of lighting fixtures. But you can use regular fake plants. I do still recommend silk/fabric plants over plastic plants as plastic plants can cut up fish fins and that's no good! So lots of plants of all sorts and some nice hiding places like you have now. The one is good but two or three is better
I know this is all a lot of money but fish keeping isn't cheap, I'll say that right now. If it is possible, can you find the Parrots a different home? Maybe someone who already knows and keeps Parrots or bring them back to the store? You could sell them too, I'm not trying to get you to just give them away since you did pay money for them and all!
I hope this helps and please ask all the questions you need, even if you think they are silly! Any question is valid ^_^
You cannot keep them in that tank unfortunately; they get much too big! Have you ever seen a full grown Parrot before? I can't remember exactly how big but each fish needs at least a a 132 liter but a 200 liter is much better for ONE parrot fish, not two parrot fish. So if you wanted to keep them you could upgrade your tank to a 200 liter and that would be totally fine! But if you are limited to that size tank then I suggest you move them out.
I mean, you can keep them in there if you really want to but it won't end well for you or the fish. They may get along in the end but more than likely that bigger one will find out that the little one makes a nice tasty treat one day and will kill him for intruding on his space! That would be pretty sad to happen :-(
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