Sick sword tails. Please help

My swordtails have bubble like lesions on their tail fins. I got the best pictures I could.

The fish in the photo also has very severe fin rot. In fact, there looks to be no tail fin left.

We need more information to help you.

How big is the tank, volume and length?
How long has it been running?
Does it have a filter and heater, and what is the temperature?
What fish are in the tank - species and numbers?
What are your test results for ammonia, nitrite, nitrate and pH?
Is your tap water hard or soft? We need numbers, please. Your water supplier's website should have that information.
What chemicals do you add to the tank eg dechlorinator and anything else from a bottle?
Have you added any new fish, decor, plants etc recently?

gas bubble disease.

How long ago did it appear?
Did you do a big water change or temperature change before it happened?
Do you have a power filter or powerhead with venturi on the tank?

It just appeared recently. The tank is a 20 gallon. I have a hanging filter on the back. I replace the the filters once a month. I do 25% water changes once a month also. I haven't added any new fish, rocks, plants or decorations. I have hard water ( well water) I don't add any chemicals. There are 4 swordtails and two guppies. The swordtails are about 3 months old. I am not sure on the nitrates or nitrites or ammonia. I have had tropical fish all my life. I have never seen this before. The temperature stays steady between 74-76. It has a heater. I am not sure what venturi is. Also only the swords seem to have it. The guppies are fine. They are all very active and eat well

First thing is do not throw out or replace the filter material each month. If you have sponges and other filter media in a filter, it develops colonies of good bacteria that convert ammonia into nitrite and nitrate. If you replace the filter materials each month you are removing the good bacteria and you will have to wait until more bacteria grow on the new materials before you have an established filter.
Without an established filter running on the tank, you will get ammonia building up in the water and that will harm the fish and cause finrot (where the tail has disappeared).

If you have carbon (black granulated substance) in the filter you can throw that away and replace it with a sponge. If you want to use carbon you can but it absorbs chemicals like plant fertiliser and fish medications from the water and is not normally necessary.

Filter sponges should be squeezed out in a bucket of aquarium water. When it has been cleaned, tip the dirty water onto the garden, and get another bucket of aquarium water to clean it again. Then put the sponge back into the filter. The filter case and motor should be washed under tap water to remove any gunk that is in it.

You probably have ammonia in the water and should do some big water changes to dilute it. I would do a 75% water change and gravel clean the substrate every day for at least a week. Reduce the feeding to once every second day and try to get the water tested for ammonia and nitrite. If you don't have test kits you can take a glass full of tank water to the petshop and get them to test the sample. Write the actual results down on a piece of paper and post the results here for us to see.

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A venturi is simply a water pump that sucks in some air so you get air and water coming out of the pump. Hang on the back (HOB) style filters do not have a venturi so that is not an issue.

Sometimes venturis can create very fine bubbles that fish absorb through their skin and it can cause similar problems to what your fish have. Very fine leaks in sealed canister type filters can also do that. You have a HOB filter so that isn't causing this.

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Do you aerate the well water before you use it?
Well water can be clean but stagnant and it may have no oxygen in it. If you add water to an aquarium and it has no or low oxygen it can cause problems.

Do you let the well water warm up to room temperature before you use it?
If the well water is cold and you add it to a tropical aquarium, it can cause a massive change in pressure around the fish and that can cause gas bubbles to appear under the skin.

It would be best to bring the well water inside in buckets, aerate the water for at least 30minutes or preferably 24 hours, and then use it in the aquarium.

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The other thing that could be causing the bubbles is a bacterial or protozoan infection. But the area is not red so bacterial is unlikely. And I do not know of any protozoans that cause the skin to blister.

Do you drink the well water and have you had the well water tested lately to ensure it is suitable for drinking?
It may have been contaminated with something and that has caused the problem. The guppies might simply be more resilient or less affected by whatever caused the problem.

. I do 25% water changes once a month also

Click to expand...

You should be doing 75% a week, yes week.