"Pandemic" in my tank

[missybounce]

I have a 400L tropical planted tank. Up until a few months ago, it was filled with approximately 30 female betta fish, a dozen guppies, 20 neon tetras and 3 dwarf chain loaches. Over the space of a few weeks, all of the tetras disappeared and next one of the loaches passed away (it appeared as though its eyes had been eaten and then it died the next day). Next the bettas appeared to disappear - hiding in plants and only coming up for air and food, and the numbers are dwindling slowly. They appear to be dying from dropsy. One appeared to have a hole in it. At a guess I would say there are only 15 betta left in the tank now, although it is hard to say as they are hiding. The remaining guppies and loaches are happy and active. Water parameters are all good, so i'm stumped at why there is a disease that seems to be attacking only certain fish. Can anybody suggest what it may be, so I can provide targeted treatment? TIA

 

[Autumnsky]

400L is ~ 105G fyi

How long has the tank been set up?

Did you add new fish / plants / decor in that time frame before the losses started?

Are you familiar with the nitrification cycle? What did you do to cycle the tank?

Do you have a liquid test kit?