Have you checked the water quality for ammonia, nitrite, nitrate & pH?
If yes, what are the results in numbers?
Can you post a picture of the sick fish so we can identify the actual problem?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
Fin rot is normally caused by a dirty environment (gravel, water, filter) that damages the fish and allows bacteria to get into the tissue. Doing big (75%) water changes and gravel cleaning the substrate every day for 2 weeks will normally fix the problem. Doing small weekly water changes won't do anything to help fix it.
*NB* Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
You do water changes for 2 main reasons.
1) to reduce nutrients like ammonia, nitrite & nitrate.
2) to dilute disease organisms in the water.
Fish live in a soup of microscopic organisms including bacteria, fungus, viruses, protozoans, worms, flukes and various other things that make your skin crawl. Doing a big water change and gravel cleaning the substrate on a regular basis will dilute these organisms and reduce their numbers in the water, thus making it a safer and healthier environment for the fish.
If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.
Fish live in their own waste. Their tank and filter is full of fish poop. The water they breath is filtered through fish poop. Cleaning filters, gravel and doing big regular water changes, removes a lot of this poop and makes the environment cleaner and healthier for the fish.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media in a bucket of tank water.
Bettafix, Melafix & Pimafix all contain teatree oil, which can leave a film on the surface of the water and prevent labyrinth fishes being able to breath. It can also prevent gas exchange at the surface.
Make sure you have lots of aeration/ surface turbulence and good filtration if using any of these products.
I would try daily water changes and gravel cleans first. If the problem doesn't improve after a couple of big water changes, then treat the tank.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge. Do a 75% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.