OP, you should be concerned about more things going wrong with your current set-up than HITH disease.
Honestly, with your current stocking levels, your tank is going to eventually fall apart. If it doesn't, that doesn't mean you are right and everyone here is wrong, it it just means your fish are unfortunate
to be hardy enough to survive in what is a totally inappropriate environment.
Have you ever seen a full-grown pleco at all? A full-grown common pleco probably wouldn't even fit into a 5 gallon tank. Bristlenose, which are the second most commonly sold species of pleco are not exactly small either. The one I had was at least as long as my hand.
I can't imagine a filter made for a 5 gallon tank that would have the media capacity to deal with the bioload your tank currently has.
I am not even going to touch on the inappropriateness of housing male/female splendens together. Luckily for you, your bettas will probably all be much too stressed to concern themselves with fighting.
What you are doing is animal abuse pure and simple. You can dress it up any way you want, use whatever excuses you have to, to make yourself feel better, but you are
abusing those fish.
I see you posted about the 30 gallon. Unfortunately, those fish need that tank now, not in some distant future.