You could get the fire eel, in the oddball fish index I wrote an artical on keeping them if you need more info.
Things like asian aros would work, greens are the cheapest and you can get them for about £100. Another option would be a black south american aro which can be bought for about £80 ish, but theyre harder to get.
Other things like delheizi bichirs are an option, aswell as ornate bichirs aswell.
Ornate pims, sterogeon cats, blochii pims, lima shovelnoses are all catfish options, check the catfish index and see what you like. Catfish wise there are loads of options. Check the stocklist of your local and see what they have because there are lots of options.
Other things like south american cichlids MIGHT work, but check first because I dont know too much about oscar compatability. If the oscar will be ok with most of those typre of things then you could get firemouth/s, convicts, juparis, geos theres quite a long list, but with the smaller ones, like firemouths and cons, it might limit keeping other things, like cats and aros.
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