Right, here comes a complete contradiction of MadGaps advice, IMO your stocking is abysmal!

I'm not sure where you researched but whoever provided your info quite probably deserves a fairly adequate slap upside the head.....
Cherry Barbs should be kept in groups, in good water conditions and IMO in a larger tank due to their being so active.
As has been mentioned the Common needs rehoming a.s.a.p, they get huge, main point about them i'd have as concerns your current tank is the amount of waste they produce, but i'll get to that in a sec.....
Betta are risky to keep with guppies, tetras, barbs due to various different reasons that while not always applicable (depending on the individual fishes preffered behaviours) aren't really worth risking.
Tetras are extremely sensitive,
i'm guessing this is a fairly new tank for my own reasons, am i right? How long has it been set up? Is it cycled?
Tetras again as has been mentioned should be kept in groups and will get stressed without enough companions to provide security.
The glass catfish should be in a group, get too big for this tank size and need perfect water conditions, if the current situation isn't rectified i guess he'll be one of the first to go.
I don't know enough about livebearers to comment, nor frogs although i'm fairly certain frogs produce quite a bit of waste for their size.
I'm no fish-fascist, i overstock myself, i appreciate people have different ideas and methods of fishkeeping but at the end of the day your stocking is so horrible (imo) i feel like doing a goose-step.
Please get your large tank cycled and remove all the fish excluding the Betta, the frogs and possibly the Guppies.
Theres not enough symptoms described to give an indication as to what (if any) illness is present, but i'd certainly think it is related to your stocking level and the enormous amount of waste you must have in your tank.
Keep up regular water changes until further symptoms are shown, also consider adding a general tonic type med which may be worth a crack.