Hi and welcome to the forum
If you post pics of the other rainbows we should be able to ID them for you.
What is algol and ektocrystal?
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In image wa0007 the fish has a dark patch on the gill cover (behind the eye) and there is a milky white edge to the dark patch. Is this the issue?
Same picture the fish appears to have a black and red thing between the dorsal (top) fin and the tail. Is this on the fish or was the fish in front of something black and red?
There also appears to be a couple of small white circles on the body near the black line that runs from the dorsal to anal (bottom) fin.
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The gill cover appears to have an ulcer, which is caused by a bacterial infection dissolving the gill cover. You generally need anti-biotics to treat ulcers in fish and they sometimes work and sometimes don't. You might be able to find a medicated fish food (for goldfish ulcer disease) that contains anti-biotics and you feed this to the fish 3 times a day for 2 weeks. Medicated food will not affect the filter bacteria and can be fed to all of the fish in the aquarium, or you can isolate the fish in a separate tank and feed & treat it there.
If you use Anti-biotics in the water, they will wipe out filter bacteria so sick fish should be treated in a separate tank. If you have a spare aquarium or large plastic storage container, you can move the fish into that and treat it with anti-biotics. As to which anti-biotic you use will depend on what is available.
You can take the fish to the vet and get them to take a swab of the area and look at it under a microscope and then prescribe anti-biotics suitable for the infection. However, this will cost a lot of money.
You could euthanize the fish. A lot of fish that develop ulcers have an underlying issue like Tuberculosis (TB) and they continue to get more ulcers before dying. Angelfish are not renown for having TB or ulcers but they can still catch TB and develop ulcers. The ulcers themselves are caused by various bacteria (not TB) that get into a small wound and spread. Fish living in the wrong water conditions or that are being bullied or attacked, or those with a weakened/ compromised immune system are more likely to develop them.
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If you use anti-biotics you will need to treat the container/ tank each day for several weeks. You normally continue treatment for 1 week after the infection has gone and the area has started to heal. Each day you wipe the inside of the container down and drain out all of the water and replace it with clean dechlorinated water, and then retreat the container with a full dose of medication.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
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While you are treating the angelfish in another tank/ container, you should do a 75% water change and gravel clean the substrate (in the main tank) each day for 2 weeks. Clean the filter too if it hasn't been done in the last 2 weeks. This will help dilute any bacteria and other disease organisms in the tank and reduce the chance of the ulcer bacteria infecting other fish.
After 2 weeks of daily water changes you can go back to doing them once a week.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
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You can add a vitamin supplement to the fish's diet each day to help them fight the infection. Some pet shops carry fish vitamins. If these are not available then you can use bird or reptile vitamin supplements.
Get a dry powder supplement rather than a liquid one. The liquid supplements go off very quickly.
You can crush up vitamin C tablets and sprinkle a bit onto some frozen (but defrosted) food and feed the fish that a couple of times a day. You can also give them green feed in the form of crushed or finely cut up aquarium plants or dark green leafy vegetables (spinach & silverbeet). Make sure they are clean and free of chemicals.