Doesn’t look like a modern strain of Flowerhorn. Looks like a classic Red Dragon and a healthy and hardy strain. I suspect the current condition is from poor husbandry which given enough time no Flowerhorn or even non-hybrid cichlid is immune too. OP has to keep up weekly waterchanges and at a high enough % to keep nitrates under 20ppm where currently it is at 40-80pmm even after doing weekly waterchanges for a few weeks. I agree, in the odd circumstance where the nitrates are let to get this high for a short time it’s nothing a grown healthy FH can’t handle but for the nitrates to be so high AFTER upping the waterchanges to weekly once OP recognized the loss of appetite, it’s possible the nitrates could of been even higher for a while. A grown (and well fed based off the first pic) Flowerhorn alone already will most likely require weekly waterchanges to maintain water quality but one housed with a pleco, I’m assuming a common pleco, will surely need atleast once a week, better twice a week. I recommend OP, you keep testing your water as you adjust your waterchanges schedule to keep your waterchanges atleast under 20ppm and again switch back the hikari pellet. Definitely trash the okiko. You may need to do 2-3 large waterchanges a week for a couple weeks to get his appetite back and healthy again.