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Really u r great bro. How u see the minute things in them. As u said that the flowerhorn is preventing her eggs from the oscar. Unless they will not fight like this. Its the right time for them to fertilise the eggs. Without fertilising the eggs. they are fighting.Yes I have to agree 75 is way too small for these to work long term.
One thing I noticed in the video I can see the flower horn has its tube extended but not the oscar. This tells me they are not breeding the female just laid her eggs. In her mind she is trying to defend them against an intruder. When she fully matures the oscar will probably suffer.
Please do not take offense to people voicing their opinions especially when you are getting sound advice. Myself and Stanzzzz7 are trying to help. It's your fish and your tank. In the end you can do what you want but an oscar as big and bad as they are will be no match for an angry flowerhorn.