Thank you so much for this helpful comment! I just recently started learning how to ~actually~ take care of my fish recently because i started a planted tank. i bought a gravel vaccum last week so i will definitely make that apart of my weekly routine. your suggestions are great and i will definitely be trying those out in the next few days.Hi there! First of all, your betta is gorgeous! Second, it's a he.
Just from what his fins look like, it seems like they got caught on something in his tank and ripped. If you have any plastic decorations in the tank, that'll typically be your culprit. If you don't want to use live plants, then silk plants are typically a safe and pretty alternative. Otherwise if plants aren't the culprit, a good test would be to take out your items and run a nylon hose over them. If it snags, it will rip your betta's fins. Otherwise he looks pretty healthy to me. Then again he's a rosetail, and I usually have a hard time spotting issues like finrot on them. But to my knowledge I don't see any signs of that. So I'm pretty sure all it is is a bad tear.
Now that he's got an injury, I want to address something else. You mentioned you do a 100% water change every few weeks and rinse out the substrate. I'm sorry to say, but that's not enough to keep your betta healthy, especially in a smaller tank like a 3 gallon, and especially while he has an injury. If his water is not clean, he can and will develop an infection which could potentially become fatal. Larger tanks can go longer without a water change, but smaller tanks do require more work to keep healthy. Typically most of us will do a 25% water change every few days to once a week, and maybe a 75% water change once a month on top of that if needed. Also with every water change you should be vacuuming the substrate. I personally don't like the vacuums so I just use a turkey baster to get all the poops and leftover food off the substrate. This is important because if the waste isn't sucked out of the tank on a regular basis, it will cause the ammonia levels to spike. This will essentially make the water toxic to your betta, which can cause a myriad of problems. If he's sick or injured, it's even more important to keep his water clean. 100% water changes aren't necessary especially if you're doing smaller percentage changes more frequently. The water will have good bacteria in it that will help purify the water in a sense and protect your betta. Having 100% new water in your tank means you have less (if any) good bacteria, and any level of ammonia or nitrites can be toxic, even when the tests say it's safe. It's only "safe" at those levels if the water has been through a nitrogen cyce. Same goes for the filter cartridge. That will be jam packed with good bacteria, no matter how nasty it looks. In case you're changing the filter cartridge often, it's better to just rinse the icky stuff off and put it back in the filter when needed. You only switch out the cartridge when it's so beat up it's basically unusable. Reusing a filter, especially when you're doing 100% water changes, will make all the difference.
There are more things that you can do and options you can choose from to make sure he doesn't develop an infection while he's healing. First and foremost I'm sure most people will suggest buying yourself some Indian almond leaves, or rooibos tea. Plopping some of these in your water will act as a safe and natural antibacterial and antifungal, and it's very effective. It also seems to have a calming affect on bettas. A lot of people will have this in their tank even when nothing is wrong with their betta. It's just super good for them. But it will turn your tank water a tea colored brown. Some people prefer this because it matches a betta's natural environment. Others don't like the color.
If you don't want the color, you can try something like Kordon Fish Protector. It's not a medication, which is something I personally like about it. (Medications can often be too much for bettas, and can sometimes cause more harm than good. I'd prefer a non-medical way.) The Kordon Fish Protector is packed with a lot of vitamins and (I think) amino acids that are awesome to a betta's health, especially when he's injured. It also adds a slim coat to the water which will serve as a protective layer on the betta that aids in healing as well as being a kind of soothing agent. I put this stuff in my water even when my betta is healthy. It's just super good for them. Heck, you can use this with the Indian almond leaf or rooibos tea if you want!
There are others who use other methods too that I'm sure will chim in. You've got some options. But these are my tried and true methods.
Hopefully his beautiful fins heal up soon. Rosetails are one of my favorites, and you've got an absolutely gorgeous one!
do you order almond leaves online? and if so what is a credible source?