The same thing that protects fish that get over a bout with ich on systems where there is no UV running. Their immune system.wings":1szn6jqz said:Finally, if it wasn't UV that protected my fish - which were covered in crypto - what was it that did in the quadradic breeding in a closed system? If you're going to disagree that my UV didn't kill the cycle - you need to tell me what did.
Fish can develop an immunity to ich after being infected one or more times. This has been studied, although I can't say I know much about the details. My understanding is that the immunity has been shown to be temporary, thus at some point in the future the fish becomes susceptible to infection again.
Also, as Buda pointed out, you should read some of your own sources - they're not really supporting your argument. :wink:
Anyway, all that said, the UV may provide some benefit here (as well as a few others), and personally, I've never seen any negative effects from using one, so I'm not against them. I'm just against telling people not to quarantine or properly treat fish parasites and to just "strap on a UV" instead.
In some settings, like commercial holding systems, they make sense to help prevent the spread of bacteria/parasites between holding tanks, but are not a total cure within an individual aquarium (thus the loss of popularity in the home aquaria application).