It's a bit off, looks semi transparent in the upper part.
Monitor it over the next few weeks and if it continues, then deworm the fish.
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Rainbowfish need plant matter in their diet. They will pick at algae, Duckweed and most soft leaf aquarium plants.
You can use goldfish flakes and pellets, vege flakes/ pellets and plants based fish foods for them.
At least half their diet should be plant based.
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When you feed fish, offer a little bit of food and let them eat it all. Then offer a little bit more and let them eat that. Continue doing this for a couple of minutes or until the fish are not rapidly attacking and eating the food.
Remove any uneaten food after feeding.
Don't use frozen bloodworms for rainbowfish. Some brands cause problems to them (internal bacterial infections). Use frozen brineshrimp, daphnia, mysis shrimp, marine mix or prawn instead.
Don't feed rainbowfish tubifex or blackworms because they cause the same problems as frozen bloodworms.