i used to breed bettas. So i had to prep a lot of tanks.The best way to paint the tank ( i am assuming it's glass) is to take fine wet/dry sandpaper and lightly sand the surface to be painted. Roughing it up helps the paint to stick. I then would tape off what i didn't want painted and used spray paint. Flat paint is soft and hi-gloss is has a harder finish. Having said that I prefer an eggshell finish. It's harder than flat but not to glossy. I would do at least 2 coats. And use an acrlyc.
Latex has a tendency to peel. Espcially if it gets bumped or scratched
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