I think my betta is sick, can anyone help me out?

I'm sorry for the long post, but I've tried to include as much relevant info as possible.

I've had my betta Salem for about 14 months now. I got him from a regular pet store so I don't know his exact age, he was fully grown when I got him though. He hasn't seemed so healthy over the last few weeks and it seems to have gotten worst in the last week.

First of all, the tank info. It's 30 litres, with a filter with a spray bar, and heated (25 degrees). Planted, with root tab fertilizers only. Three hiding caves (a smooth plastic cup, a smooth clay tube, and a floating log). No tank mates except a zebra nerite snail and some general pond snails.

I feed him 3 Hikari betta pellets once a day, with one fasting day per week, and usually feed him frozen food twice a week (generally bloodworms, brine shrimp, or daphnia).

I've tested his water a few times this week and each time ammonia and nitrite were both 0. Nitrates are 40, unfortunately this is the level of nitrates in my tap water. pH is 7.4. For the last few weeks, I've been changing a third of the water every 2-3 days. Before that, I did this once per week. I use Prime water conditioner.

The symptoms he's showing:

• fin deterioration, this has been a problem for a few months now, it's fairly minor but it won't seem to stop. First I attributed it to excessive flaring, then to fin-biting, now I'm thinking fin rot. No black edges to the deterioration, in fact he used to have an entirely black edge to his tail that was his natural colouring but now that's almost all gone. Photo's not great but I hope it shows it a bit. photo

• He's become quite lethargic, he's spending a lot more time resting at the bottom or inside his betta log than before. He's also stopped flaring (he used to flare any time the light outside the tank was slightly too dim or at the wrong angle, it was a bit of a nightmare) and hasn't made any bubble nests for some time. Despite this, he always comes swimming up to see when somebody walks up to or past the tank.

• Constipation, seems to come and go every couple of weeks. Each time it reappears, I fast him for 3-4 days then feed frozen daphnia for a few more days. He still has a good appetite and eats all the food I put in.

• A greyish/pale patch has appeared on the bottom of his face. Not fuzzy or anything like that, it's just discoloured. Also, the red vertical stripes on his gills seem to have gotten brighter.

• He seems to be breathing a bit heavily/more rapidly than normal, and his gills don't seem to close all the way. I'm thinking they might be inflamed? However, he's not gasping with his mouth, and he's not spending time at the surface (except when I go over there and he's expecting to get fed). I've got a couple of (very short) videos to show what I mean (please excuse the dirty glass and my shaky hands!): video 1 video 2

So all of these symptoms would suggest a water quality issue, however my water is always testing fine (and I am certain my test kit is working as I'm also fishlessly cycling another tank at the moment, and getting ammonia and nitrite readings when testing that) and my tank has been cycled for over a year.

Plus in that case, surely the increased water changes should be helping? However they don't seem to be making any difference, in fact I'm wondering if the stress might be making him worse - he was fairly active yesterday, when I hadn't done a water change for three days, in comparison to today, after his water change this morning he's done nothing but sit at the bottom apart from the times I've walked up to his tank. Should I continue doing the water changes? :/

I'm wondering if it might be a bacterial infection of some kind? I don't know though, I'm very confused and worried, so I hope people have some suggestions for treatment options I could try. Thank you so much!!