New to the hobby and forum.

  • We are currently upgrading MFK. thanks! -neo
Looking good I would add some larger plants to break the sight of the other fish. Just in case it'll keep aggression down.
 
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Welcome to the Fishy family.

Pretty simular setup, but I also run Aqua Clear 110s.

Ideally you want your filtration to turn over your tanks volume of water anywhere from five to ten or more times an hour, depending on stocking levels. More fish more turn over due to more waste being produced.
Manufacturers tend to test thier stuff with no media in them to get "a better number" so I take away 30% of thier listed flow rate.
Your FX4 flows 700 gallons per hour. Minus the 30%=490÷125=3.9 turn overs per hour. Should be fine for now but as they grow it will need a lot more.

As far as the fish goes, Oscar's are South American, the Jack Dempsey and Fire mouth are Central American. Normally people don't mix the two due to different aggression levels, water needs.
The Firemouth are more like tetras as in they need a group of 5 or more to be comfortable.
The Oscar's if kept in good water and fed good food will grow an inch a month for the first ten months or so. They are really dirty fish as in how they eat, with so much uneaten food flying out of their mouth and gills. They urinate and poo just as much. With mine I did water changes every three days, till it got bigger then changed to every other day.
I would choose either the Oscar's and pleco, or the JDs, Firemouths, pleco. The Oscar's will out grow everything else and eat or torment the rest to death.

A 125 is a good home for a single Oscar, pushing it with a pair. Now the JDs it a much better home as the male's get 10in females smaller yet. So comparatively the JDs would have more room vs a pair of 12in Oscar's.
The pleco is probably a common so gonna grow to about 16in as well and makes copious amounts of poop.

You have some time to pick what you want. About six months tho the JDs will mature and can breed maybe tossing a whole new lvl of aggression to the tank.

Oscar's take 1.5 years to mature, they put all thier size on first before they breed. So need a bigger tank really if you want them to breed, 6x2x2 or an 8x2x2 preferably.
 
Thanks for taking the time to explain all of that. Kind of thinking maybe do away with the pleco and oscars which should allow for more firemouths or maybe a pair of other cichlids. This is all new to me. I like oscars but dont want to dedicate this whole tank to one or two fish. I would rather be able to have more like 8-10. For now they are all very small so hopefully i can have a luttle time to decide what to do.
 
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How about keeping the firemouths and 1 of the jack Dempseys , get rid of the other fish, then add the following:
1 pair of mellenium red rainbowfish
3 to 5 orangespotted sunfish
1 longfin golden bristlenose pleco
 
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