I have two aquariums and only one is set up for display the other was just there. I had two oscars, two convicts and one other fish that I have had for years. I have recently had to remove all other tankmates from the general tank and set-up the spare due to and unexpected batch of convict fry leaving only the convicts and their young, but one of the oscars have already died due to bullying by my older fish (whom I have isolated in a smaller container inside the aquarium), the remaining oscar looks to be ok which is surprising being that the two were inseparable but I dunno, as long as I have been keeping fish oscars are a first and after reading up on them I fear that I am not properly equiped to keep him happy. How do I know?

first off how big is your primary and secondary tank? secondly what kind of fish is your older one?

you will have to decide on what fish you are wanting to keep. a breeding pair of convicts can be extremly aggressive to any other tankmates and should be kept alone.

depending on what type of fish your older one is it might not be compatible with your oscar and might have to be kept alone as well.

also how big is your oscar? they do grow rather quickly and if he is still small he could outgrow the aggro older fish and it might not be a problem in the future.

Your best bet is to have a good read of all the information on the site. OFL has done a good job with this. If after reading all the articles you feel that you cant do a good job of keeping an oscar then rehome it. Oscars are not that difficult to keep, as long as you have a big enough tank and good equipment and are prepared to put the effort in. They are worth the effort

my primary tank is 55 gallons and the secondary is about 35 gallons. The convicts and their fry will be left alone as I have already split them and I do know they can be very aggressive even when not breeding, so I had to move the others. My older fish is some kind of cichlid that I happen to think is some sort of mutt. I research every fish that I keep and that one is impossible to find but I do know that he is in the cichlidae family. I have also thought of the fact that the oscar will grow and then be able to defend himself but it is the wait for that to happen that is the issue right now he is a baby (immature), but I haven't measured him yet. I dunno I suppose I will have to wait this one out the biggest problem is I dunno if the petshop will take him back.

Most shops round here will take a fish and some will even give a credit for it, however the local big chain will not take fish back. It would probably be a good idea to find out if they will take fish back, that way you will know your options

Your 55g is big enough for only one oscar, so you need to start sorting out what you are going to do, which fish to return or getting another tank for your fish