My fish are dying of cancer.

WoW............ I never heard of a fish getting cancer, although my dog has it......

 

You sure it's cancer? Could be parasites. Cancer in fish rarely gets big enough to be seen before it kills them. Got a pic? What kind of fish? (I am betting goldfish, and you will see why)

When they start to get sick do they swim left only? (and I am serious with this question too)

There are three things that can cause it if it is cancer.

Something in your water. In which case I wouldn't let your water touch my bare skin. Doubtful, switch to bottled water to fix.

Something in their food. Since 99% of all fish food is made of fish meal that was made from fish for human consumption, doubtful. And that could be the problem. (see below)

Aerosol cleaning supplies. Furniture polish is a biggie, glass cleaner too, oven or bathroom cleaners I would put highest on that list.

However. I am going to guess it is dropsy.

http://www.wetwebmedia.com/CypriniiformPIX/Goldfish/Goldfish_with_Dropsy.jpg

Dropsy is malnutrition in a fish. 99% of fish food is fish meal. Which goldfish (for instance) will eat willingly. Problem is, goldfish are vegetarians. So they get special vegetarian mixes. If you have goldfish, try this.

Get a veggie clip from the store. It is a plastic clip with a suction cup on it. Then get next time you are at the grocery, check the asian food aisle and find the dried seaweed sheets. Ingredient list should have nothing but seaweed in it. Throw that in the clip and see how they like it.

No matter what the fish type, toss out whatever you are feeding them and switch to something else.

 

Do the scales stick out? Like a pinecone?

 

I am still thinking bacterial infection.

It would infect all fish eventually. It causes the body to bloat without lesions. Plus because it is fluid, the fish swims and acts normally. Swim bladder infections cause the fish to have trouble controlling its buoyancy.

Are we talking about a single puffy spot or an overall bloat? Like one side sticking out. Or are there actual lesions?

A picture would really help. But I know how hard they are to photo.

PimaFix and MelaFix would be what I would try first. They are the 1-2 punch that everyone swears by.
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=11269

 

Those are most likely Neascus Digenetic Trematode.

It's a parasite. The fish grows this black gross looking cover over it as a reaction.

PimaFix and MelaFix, follow the dosing. You might even do a salt water dip to start. However that can be a little hard on a weak fish. It is very effective though. Think salt on a slug.

Couple days the Pima/Mela mix should start to show improvement.

To do a salt water dip, do a water change but keep the water. Get the smallest size of marine salt you can at the pet shop. Then, whatever dosage it says to use per gallon, quarter it. Net the effected fish and put him in the salt dip for 2 minutes. You can go longer, but I wouldn't. Plus the dip doesn't get rid of them in the tank. (They probably have a nymph cycle in the substrate)

Remove immediately if it shows any sign of distress in the dip.

Freshwater parasites can't handle salt water and vice versa. However freshwater fish lack the membrane that salt fish have that keeps the saltwater from dehydrating them. Hence with the open lesions, it would magnify the effect. So with a fresh in a salt dip, the parasites get all the moisture pulled out of them, like a salted slug. With a salt in a fresh dip, the sudden drop in density causes the hard chitenous bodies of the explode like they were in a vacuum.

 

Oh, and they come in with snails.

 

Luckily, Pema and Mela are pretty safe. One is like a tea extract of some kind. The other is made from nuclear waste. They balance each other out.

 

tea and nuclear waste cancel each other out...? thats so bizarre

 

Just like pirates and ninjas.

 

now..it all makes sense..*nod*

 

OK one is a tea extract and the other a bay extract.

Hey, when I was a kid I played with mercury... a lot. Foulest tasting stuff ever.