He needs cleaner warm water. That's the key to his recovery. As long as that's not address it is impossible for his fins to heal.
2.5g requires 2 water changes a week 50% 100%.
When you reach 5 gallons you can do only one 100% a week.
If you want to add a filter to the 5 gallon or go even bigger with a filter you need a drops kit for ammonia, nitrite and nitrate. They need to be tested and monitored daily
and a water change needs to happen every time you see ammonia or nitrite hit .25ppm. In addition to this one 50% weekly water change will be required. Once the tank is cycled you'll only have to do about 25-50% water change a week (depending on how large the tank is) but cycling could take up to 2 months this way. http://www.bettafish.com/showthread.php?t=47838
There is no substitute for water changes and fin rot happens because of poor water quality.
You see on the package it says fish meal? That's fish byproducts ground up. Right after that you see wheat. In a good pellet you should have the first two or three ingredients as whole fish. You should see his poop on the bottom and it should be obvious when you do the 100% water changes and clean the gravel.
So to get him better
* Clean his bowl and up his water changes (be careful about acclimating)
* Get him heated up (acclimate)
* Look for the antibiotics or you can also use aquarium salt, predissolved at 1 tsp per gallon along with very frequent water changes. AQ can only be used up to 10 days. It will not leave the water and will only be taken out with your water changes and you must replace it along with your water changes. http://www.petco.com/product/1697/AP...t=OnSiteSearch
no you know you're in the clear when his fins stop receeding and you see some white growth on the tips.
* Adjust his diet and monitor his poop