Strange lump on BN head?

Tank size 50L (Juwel stater tank), plenty of places to hide w/ bog wood, fake plants.

Substrate: Small pebbles

Filtration: Internal, cleaned today

Airstone/sponge filter etc: No airstone, just plain sponge filter nossel causing surface disturbance

Inhabitance: 3 Adult BN (1 male, 2 female), 1 fry BN (buggers won't stop breeding, missed one of the eggs.), 3 Kuhli Loach (introduced today, was told by the pet store that this wouldn't be overpopulation, but I didn't know I had the BN fry at the time, today was full of surprises)

Fish affected by issue: Just the male BN as far as I can see. He's a few years old, 9cm long. The others are too shy for me to check...

Feeding system (what, how much, how often): 2-3 Wafers daily (I don't feed them zucchini as often as I should)

current water change system (before illness appeared): Weekly/fortnightly water changes w/ vacuum

As many levels as you know: pH 7.6, Ammonia 0ppm, Nitrite 0ppm, Nitrate 40-80ppm (can't read chart very well). The nitrate I've just become aware of today and am trying to get on top of it, in the past they've always been 0ppm. Prob accidentally overfeeding them caused it?

Major changes in the last 3 weeks: I had a major snail infestation (hundreds of them) and did a full clean of the tank. I don't know if nitrate was an issue at the time but I've never had any in the tank before, so it's a sudden shock for me. (i've been a bit slack on water testing and changing lately though. My dog recently went blind, work got busy, yadda yadda, and the poor fish have been a bit neglected) Also I have been struggling to keep the temperature down, summer is approaching and the tank is edging up to 28-29C no matter what I do (fans and ice have not worked in the past, anyone got any tips for that too?)
I've had 2 BN and 1 Betta die unexplainably but that was months ago, my 3 BN have always been so hardy I never expected to see anything like this.

Symptoms: Weird lump on his head near his eye (really obvious in photo)... otherwise he's acting completely normal.