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Ich is one of the most common illnesses in the aquarium. Many if not most aquarists will experience an ich infection within their first 6 months in the hobby. Fortunately it is very easy to prevent and very easy to treat.
Ich, Ichthyophthirius multifiliis, is an external parasite commonly found in aquariums. Like many illnesses, ich cannot infect healthy, unstressed fish. This means that when there is an ich infection something is stressing the fish. It may be water quality or a water parameter (such as pH, temp, or a sudden change in either), or it may be the behavior of another fish. Frequently the aggressive or nippy behavior of one fish can be enough to stress other fish enough for them to get sick. Keep in mind that you may never see any blatant aggression, it may be subtle or the culprit may be nocturnal. A fish swimming away from a fish quickly may be the only sign that a fish is being a bully. Just because you don't know what the stressor is or can't figure one out doesn't mean it's not there or you have some unique case where the ich in your tank can infect healthy fish. This just doesn't happen. If this were the case then every tank that had one fish with ich in it would soon have every fish infected with ich. Although I am sure there have been cases where every fish has been infected this is very rare and still means the fish were stressed. Ich just doesn't infect healthy, unstressed fish.
Ich is very easily identified by its white spits. The spots generally look like grains of salt. In some cases the spots may be larger or clump together to appear as one larger spot. I remember routinely seeing painted glass tetras with ich only around the injection sites where the dye was injected with needles. The injection site was a wound open to infection and ich took its opportunity and infected them. There was so much ich around the injections sites that it was just a giant ball of ich.
Treatment is usually very easy and effective. In most cases removing the stressor is all that is needed (correcting the water problem or removing the aggressive fish). Keep in mind that the ich infection may have been triggered by something during a water change. This is usually a sudden change in temp that you may not have realized or didn't catch until it was too late. The stress weakens the immune system which allows the ich to infect the fish. Once you remove the stressor the immune system can usually get rid of the ich on its own.
Extra water changes are always a good idea when anything is wrong. This is especially true with ich. Unless there is something about your water changes that is stressing the fish they will only help. Do water changes every one to three days with an ich infection. Keep them the same size as usual, don't go larger.
Salt treatment is also very effective against ich. A full dose is one tablespoon per five gallons. It is best to start with a half dose and then put in the other half dose after about twelve hours. This is especially true if you have any fish that may be sensitive to salt such as loaches, tetras, discus, etc. In most cases there will be no issues but every once in a while a fish will have a bad reaction to the salt. In these cases they may become lethargic, breathe rapidly, or have clamped fins. In these cases you will want to do a water change to remove some of the salt. Goldfish and livebearers (mollies, platies, swordtails, and guppies) react very well to salt treatments. Keep in mind that you need to dose for salt again after each water change. So if you have a full dose of salt and then do a 25% water change then you need to add a full dose of salt for the 25% you changed. So if you have a 20 gallon and do a five gallon water change you need to add a full dose of salt for those five gallons, one tablespoon.
Garlic can be a very effective treatment against external parasites. I have seen cases of it being used against marine ich (unrelated to freshwater ich, but similar in appearance) in reef tanks where chemical treatments are not an option. I have seen New Life Spectrum Thera+A work to treat ich because it has so much garlic in it. In general food soaked in freshly pressed garlic is the best method because it is freshly pressed garlic that is most effective against parasites. Bottled garlic products sold at fish stores will not be effective against parasites. For more information about garlic please read: Garlic and its Ability to Kill Parasites
In my experience hospital tanks are not the best method of treatment. If one of your fish has ich they are all exposed, so isolating the visibly sick fish is not going to benefit the other fish. Hospital tanks are usually uncycled and smaller than the display so the water quality tends to crash very easily. Sticking an already stressed, sick fish into a smaller, uncycled tank is asking for a crash and more problems. In my experience leaving the fish in the display tank is a lot less stressful on them which leads to a faster, easier recovery. For more information about Hospital/Quarantine tanks please read: Quarantine: More Harm Than Good?
Raising the Temperature:
Raising the temperature is also a very common way to help get rid of ich. The idea is that is speeds up the progression of the whole progress. So as you start to do the right things to get rid of the ich, the higher temperature helps push the ich through its life cycle and get the fish over the infection faster. Raising the temperature up to about 82F in most cases is ideal.
Fish That Are Very Prone to Ich:
There are some species that seem to be especially prone to ich, what some people call "ich magnets" because it seems they can't help but to get ich. These fish include: clown loaches, pictus/pimodella catfish (Pimelodus spp.), silver dollars, and many small tetras such as neons, cardinals, etc. When purchasing these fish it is a good idea to take a close look at the fish you are buying as well as all the other fish in the tank. However, although these fish are very prone to ich (they can be perfectly healthy and break out just from a cool water change), any fish can get ich. It doesn't matter if they have no scales, small scales, are large, hard, plate-like scales. So be careful and be aware with any type of fish.
A Demonstration of the Connection Between Ich and Stress:
Many people think of ich as being infectious and that its presence is all that is needed for an outbreak. This couldn't be further from the truth. Ich CAN NOT infect healthy, unstressed fish. The fish must be stressed. You may not be able to figure out what the stressor was, but it was there, at least at some point.
An experience I had demonstrates this perfectly. We got in a couple flagtail prochilodus at the shop I was running. I picked them out at the wholesaler, they were fine and so were the others in the tank. Shortly after getting to the store they broke out in ich. I did the usual treatment but after a few days they weren't getting better. I wanted to take them home for myself since they are one of my favorites and they are only available a couple times per year. I started to realize that the tank they were in just wasn't ideal for them. It was a sales tank, it was too clean. There was no algae for them to graze on. It was a little too well maintained. I decided that my 150 was probably a better home for them. I don't mind algae on the decor or the back wall. It was also more fully decorated (you can't chase fish around a sales tank full of rocks and driftwood, just a few pieces that are easy to pull out at the most). So I bought them both and took them home. Sure enough within days of being in my tank they were much happier and more relaxed. They could graze, had more space, more decor, and it was more natural. Within a week they were completely void of ich. On their own, without an increased temp, salt, meds, or extra water changes, they got over it completely on their own. Once I removed the stressor (the less than ideal tank they were in) they got over it on their own. In addition, none of the other fish in the tank they were introduced to ever came down with ich at all.
I have also bought fish from tanks and systems that had other fish with ich. The fish I bought didn't show signs but were obviously exposed. They never came down with ich and neither did any other fish in the tanks I put them in. In these cases I made sure the fish I bought were healthy, not only showing no signs of ich but also alert, active, erect fins, etc. These fish were doing well and were not stressed even though other fish in the system were. This showed they were hardy and adaptable, not easily bothered. Those are the fish I want.
I am not recommending that you go out and buy ich infested fish. These examples are only given to show how strong the association is between ich and stress.