Can a large Oscar fish and a small Oscar fish live together safely?

Probably not, for a few reasons. The first is tank size. Unless you have a large tank, upwards of 150+ gallons, 2 Oscars can overwhelm a smaller tank and filter with their bio load (wastes). Ideally, you want at least 55 gallons per Oscar. Another reason against it would be that if you end up with 2 males, it is likely they will be fighting constantly until one is injured or worse. Lastly, the long fins on the fan tail will invite nipping which will cause injury and may lead to infection.

As long as thee oscars are small there will be no problem, however they grow very fast and they will eat all your other fish except the gouramis. So yes, you will need a separate tank for them and preferably at least a 40 gallon tank. The same is true for the parrots, another cichlid species.

That would depend on the size of your tank and the size of the oscars. Arowanas need a lot of space usually. I have seen them successfully being kept with arowanas before, but the oscars must be at least the same size of their mouth, not smaller. Unfortunately there are not fixed ruled and the only way would be to try them out together. Hope this answer was helpful, but please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.

A 55 gallon tank is truly not large enough for an Oscar. Usually they are housed singly or in a mated pair. A single adult Oscar should have around a 75 gallon tank MINIMUM and a pair needs 100G or larger. Here's a bit more info on Oscars https://www.petcoach.co/article/oscar/

I would strongly recommend against keeping these 4 fish in one tank. Oscars can reach 12 inches when fully grown. JDs can reach 8-10 inches. You will run out of tank space eventually, or fish if they, being cichlids, become aggressive.

I wouldn't recommend keeping a smaller turtle with a larger turtle. Oscar would harass or try to eat the smaller turtle. Red eared sliders can be quite territorial and aggressive towards other sliders. If you have a huge tank, say over 100 gallons, it *may* work. If you are set on having another turtle, look for one that is similar in size. The problem with mixing species is that there may be care and environmental differences that you may not be able to overcome in order to provide adequate care to both species.

All these fish can grow quite large. The loach would be the "smallest" at about 6 to 8 inches when full grown. The Oscars can reach lengths of 18 inches and the Texas cichlid and sharks can reach a foot in length. All these fish will require much larger tanks than the ones you have now. The three Oscars alone would do best in a 125 gallon aquarium MINIMUM. The others may thrive in 55 to 75 gallon tanks. To leave them in small tanks will cause stress, aggression and likely infection.