Ray Pollett, a subscriber to our FW mailing list, reported in a series of messages that his fish were dying one after the other and that they all had bloody markings on their bodies. After several exchanges I asked him to send me a sample of an animal that he thought was the cause. He did so and after it arrived, and identifying it, I sent him the following message:
Ray
The sample arrived and what you have is one of the most carnivorous leeches in the freshwater hobby. No wonder your fish are dying. I am sorry this is happening to you.
It probably came in with plants as real small ones, but as you noticed they grow and then attach to fish leaving markings behind that look like bloodied spots. I certainly caution you to be careful of the source of the plants since they may have an infestation of them and not even be aware of it.
When they are small larvae they are very difficult to notice and have a tendency to be on the underside (non visible) of the leaves of the plants. They can come in with a fish too and still be small so you do not see them. They can reproduce asexually, hence the fact that one can give rise to a whole number of them. Ususally though when they enter the tank in some fashion there will be more than one.
Because of the damage they cause by sucking out the fishes blood, markings are left on the body and eventually the fish die due to stress and possibly lack of blood. These leeches can indeed suck out quite a bit of blood as evidenced by the size they become. All of them need to be removed as fast as you can or you will loose all the fish in the aquarium.
You can try and trap some with a trap that contains food. They will enter it at night and you should be able to get some out in the morning.
The other method is to use a freshwater puffer which will eat some of them. The third method is to pick them out with tweezers, one by one.
You cannot really add products to the tank that will kill the leaches as they will kill off the remaining fish and the plants too. This is of course unfortunate. Below are some of the methods I suggest for ridding the tank of them.
You may see more of them during the night than during the day (use red light, although once they are out they cannot really get away that fast for you not to be able to catch at least some of them).
With tweezers and the other methods you should be able to get rid of them.
Alternatively remove all fish from the tank for at least one month and keep removing whatever you see. Since there will be no hosts, they will slowly die off for lack of food (they need blood). This is sort of the method of last resort, but may very well be what you have to do, in combination with removing some of them in other ways, to rid the tank of them completely.
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