I think the thing in the middle being referred to is the marine-looking ornament hiding your air stone in a freshwater setup and the problems with the stocking is more in keeping with the types of fish you have together.
Typically your tank is a little small for 3 spot gourami and I think I see a pearl gourami in the background (behind the snail tank) and a betta splendin, male & a single female (from what I can see). In many cases, the female will get stressed by the male and 3-spots can be right £$%^ sometimes and will hate your Pearl to death. If that doesn't get it, your male betta will typically go for all its tank mates as those caudal fins look like too much of a challenge.
This is just a typical scenario for most people and yours may go no further than just stressing each other out (which could be the cause of sickness) but at the same time, there are people out there who by sheer chance mixes that normally result in death works.
Ultimately though, having the bettas in their own tank, adding another female or two and if you want something extra in the tank, try african dwarf frogs (not clawed, dwarf) which can reside really happily with bettas would be something that most fish fanatics will advise.
beyond that, I like the colours