It sounds like fin rot, which is treatable, but Goldies, even small fancies like Moors and fantails need a far larger tank, cooler water and very much more filtration and water changes than do tiny livebearers and yes, male formosa especially can become quite aggressive and nip the long flowing fins of any slow moving Goldie many time's their own size.

To treat the Goldies, get them out of that tank ASAP. If you don't have another tank, buy a heavy duty plastic storage container of at least 20 - 30 gal. capacity, fill it with fresh conditioned water and medicate for fin rot. If you do a 50% water change daily, you don't need a filter and don't want one with the meds anyway, although even if you have to run it off a small cheap pump barely capable of making more than a few bubbles a minute, you will need an airstone to roil the surface a bit.

Presuming you are catching it early, the only med. you may need is Melafix, which is a safe, natural remedy, available at any pet shop, or the pet dept. in most Wal-Mart's.

You could do easier I expect; by placing the tub outdoors in a shady spot near your outdoor water source. Then, with some 5 gal. buckets, you could dip 2 - 3 out and refill pretty quickly, dose, and give them a light feeding.

Do that daily and unless they are far more ill than only suffering from fin problems, they should pull through for you, but it may take several weeks before they are back to feeling good and months before those shredded fins have regrown.

However, if you can save the ones you have, think carefully about how you want to keep Goldies in any aquarium and as they grow, plan on at the very least; a 29 gal. tank for no more than 3 of them, better a 55, with a filter rated for at minimum- 3 X the size of the tank, with 50% water changes weekly.

Otherwise, you might be best off not trying to keep Goldies in any aquarium.

Thanks for the info. It sounds like fin rot. I will get some meds. I have not seen the Mosquito fish getting agressive with them at all. And it's really a pretty large aquarium. Probably 50ish gallons. I guess I just consider it small in comparison to my pond! What temp would you recommend for the aquarium? I was told 75-80. And once they get large I intend to relegate them to the pond. I have a lot of happy goldies and Koi in there already.

The problem in trying to keep carp in the same tank with tropicals is more often the water temps than chemistry, so while the 75-80 may be okay for the little livebearers, it's too warm for Goldies, even Fancies. I'd say 72 - 76 max.

If you are sure the fin issues are not from the formosa nipping at them, then logic dictates the only other explanation being that you bought fish that were already suffering from some type of bacterial infection that attacked their fins, so just in case; add Pimafix to the water along with the Melafix.

They are perfectly safe used together. I've used both in extreme cases in conjunction with some pretty sophisticated antibiotics and affected cures in what otherwise seemed to be hopeless cases. I'l admit to more failures than successes, but just enough successes to keep me optomistic.

In general, Goldies are pretty tough and you can often get them back in shape with just super clean water and light feeding, but once fin rot sets in, it's best to treat it with something designed for the purpose.

However, like any other meds, unless dealing with a specific disease that has been correctly diagnosed and the fish aren't yet on their last gasp, the best rule of thumb is to start with something safe and mild, and see how it goes for a few days. If that doesn't work, then move up to something stronger, but as with all meds; the stronger they are, the more easy it is to inadvertantly kill as cure with them.

So if your Goldies are suffering from a still treatable stage bacterial affliction, and most common illness in aquarium fishes are bacterial as opposed to viral; with the treatment I described, they will hopefully show improvement within a few days.

If not, then go to a broad spectrim antibiotic, such as Amoxicillin. Hopefully, it won't be necessary.

BTW: When doing daily water changes in a hospital tank, the last thing you need to worry about is cycling.

Good luck.