Betta - Possible Fin Rot?

So, I came home from work to feed my fish and noticed some new black patches on the tips of my betta's tail fins. I last cleaned his tank on Sunday morning, as usual, and he looked normal then, so it's come out in the last couple of days. I can't say for sure that it wasn't there yesterday, but I didn't spot anything wrong with him, and as I feed him some betta pellets, I usually sit and watch until he eats all of them to ensure that none end up sinking to the bottom and dirtying up the tank.

Additionally, some of his fins (mostly on the top) look a little like they're standing proud, almost like a pine cone, but not quite.

He doesn't seem himself, either. He's normally quite a lazy fish, not doing a whole lot and not even getting all that excited at feeding time, but he seems to be rushing around a bit tonight and it's almost as if something's spooked him. I don't have any kids or non-fish pets, so I can't think of anything that would disturb him, but he seems jumpy/skittish.

Nothing in the usual routine has changed. I do a 75% water change and clean at the weekend, he gets fed each evening after work and I've been using the same food/de-chlorinator all of this year.

So, tonight, I've done about a 50% water change tonight and added some API Stress Coat to see if that helps. When I checked the water before the change, my readings came as normal - 0 ammonia, 0 nitrite and 20-40 nitrate.

Can anyone confirm if this is fin rot, and if so, what I should be doing? Will daily water changes be enough, or do I need to get aquarium salts? He's on his own in the tank, so medicating shouldn't pose a problem.

Hi Sue, thanks for your feedback.

I've had him about a year and a half, and he's been on his own in that tank, in that position for about a year now. I've attached a photo of the tank, in position, in case it helps. He's got a cave, some moss balls, a synthetic grass-type plant and a rock formation. The tank's 25l, but I was led to believe that this was a sufficient size for him a year ago when it turned out that the female betta I'd brought home was a male that was starting to hassle the gourami in my main tank.

I checked the water levels tonight - 0 ammonia, 0 nitrite, 10 nitrate - and then did about a 60% water change. Just dechlorinator added to the water going in. The heater's set to 23° C, and the thermometer reads about the same, maybe verging on 24°C. It's been a fairly warm day today, so that doesn't surprise me. His tank, despite being between the two windows, doesn't get direct sunlight.

Tonight's observations are that he looks no better, and he's spending a lot of time sat in the back corner, near the heater. He did move around when I was draining the water, but he seems low on energy today. I think he looks a bit bloated, and he used to be a very dark black and now looks a little paler. He does appear to be a bit disheveled overall, and seems to be leaning to one side a little, though the bloat looks to be on both sides. I've put a box around where he appears bloated, and this area looks palest of all.

The black on the tips of his fins doesn't look any worse to me than it did yesterday.

How much should I be changing daily?

I did about 50% yesterday and more like 60% today, but is there a point where I'm changing too much?

Prince died while I was at work yesterday.

I took a good look at him last night, but beyond what I thought was swelling and the colouring on the tail/fins, I couldn't see anything else out of the ordinary in his appearance. The discolouring was no more pronounced than in the pictures I've already posted.

So sorry to hear that Prince has passed away.