Ich, or Ichthyophthirius multifiliis, is a protozoa that causes all manner of fishy itchyness. What a protozoa is, is anybody's guess. But it has an interesting life cycle and is some kind of parasite that likes fish.
Best practice would be to quarantine any new fish for a couple of weeks before adding them to your system. I aspire to second-best practice. But this isn't currently an issue because I'm not adding any new fish. I just thought you should know that.
Its a fair bet that ich colonize most aquaponics systems, fish tanks, rivers, and wherever fish hang out. Normally healthy fish seem to cope with the odd parasitic occupant, as long as the fish are not stressed, are well fed, and are generally otherwise healthy and happy. When fish are not so healthy they find it a little harder to fight off infections, parasites, and whatever else that spends it's time bothering fish.
My fish have been through a lot and, it would be safe to assume that they are perhaps not as fit as they could be. In fact its probably safe to assume that they have suffered some gill damage as a result of the poison shrub they were probably exposed to. Perhaps as a result, they are showing signs of what might be a dose of ich.
Signs of ich include fish rubbing on things, presumably trying to itch themselves, and small white growths on their skin.
Ich are up to 1mm in size, and burrow into fish skin and gills, where they live on bits of fish and blood. Once fully grown, the adult ich beastie (tomont) falls to the bottom of the tank or comes to rest on something where it builds a crusty roof over its head and spends a day or so dividing into around 2000 other smaller beasties called tomites. When they get bored and leave their crusty home they are called theronts. Theronts are elongated, can swim, and have a gland that aids in burrowing into a fish where the whole shebang starts all over again. If you're a fish, nature sure is irritating.
When the fish gets one of these beasties under their skin they do what most animals do with this kind of thing. They basically try to build a pearl around it in the form of some gristle like crunchy stuff. I'm guessing calcium and some other stuff. It doesn't matter. This fishmeat capsule makes it difficult for us to kill off the beasty, so we get it when it's doing the dropping to the bottom, and swimming around stages, when it becomes vulnerable to various chemical treatments.
If your fish are in an aquarium this means buying something from an aquarium shop or vet.
These treatments are unsuitable for fish destined for the table, so in aquaponics we treat ich with salt.
The addition of iodine-free, and anti-caking agent free pure cooking salt is used. Treatment should be at 3 parts pure salt per thousand parts water. That is a scary amount of salt to be adding to a fresh water fish tank, and takes a certain amount of courage to actually do the final adding it to the water bit.
One other way to stop the ich cycle is to deprive them of their fish hosts for 3 or 4 days. ie a fish tank left without fish for a few days will be rid of them.
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