help with crowtail plaket's dorsal fin discoloration

[Hartan]

Hey guys,
I know this might sound weird, but I keep my betta in a 5-liter round bottle - don't judge, lots of breeders do it. My little guy is 2 months old now. The tank's got plenty of plants - mostly frogbit and salvania - and I've also added some Indian almond leaves to the water. Everything was going great, my betta's super active and has grown heaps since I brought him home. But then I noticed this weird white stuff on his dorsal fin. I asked around, thinking it might be some kind of parasite, but people said no. The marks are like little circles with blue in the middle - same color as my betta. They're not holes or anything, just... weird. Anyone have any idea what's going on?

[gildfish]

Honestly, the photo's not the best, but I'll give you the benefit of the doubt. I'm not here to judge, but I gotta say, a 10-litre tank or more would be ideal for your betta. I'm really hoping you've got a filter in there, if not, please do yourself (and your betta) a favour and get one ASAP.