A hot-ish rinse with chlorinated salt water including inside/outside of lid.
Dispose of everything else in the tank that cannot be replaced easily.
IF there is a power filter you should be able to do several hot water rinses plus long-run in chlorinated water (several days on side of bucket).
Don't use straight hot tap water over 120° on the surface of the acrylic.
You CAN boil rocks, but doing so is mean on pans, cheaper to buy new.
INSPECT YOUR PRACTICES
Cats, kids, dirt, airflow, air quality.
I've had infections get into my tank that were literally cat-scratch fever, my son's stuck soapy fingers in, I clean under my nails and wash my hands twice then again with salt before I do work in the tank because I work with pesticide and herbicide.
A poisoned ant or roach will kill kidneys really fast, so will fire ants if eaten.
Do you feed by handling the food? Betta pellets are hard to manipulate without fingers. If you use hand creams, hair conditioner, pesticides, herbicides, perfumes, tobacco, air fresheners or anything of the sort you need some carbon filtration on some level. You should be careful what is on your hands when you handle the food. To minimize handling the food you can get pill sorter boxes "SMTWTFS" things with individual boxes for the days and separate out food for the fishes into a week's worth of 'pop open above tank' so you don't have to touch the food.
Half of all dropsy is water condition and has nothing to do with things put into the water on accident. Hardness, nitrate, metals -including the lead weights people use for plants-, cheap food AND special medicines add hardness to the water. If you use salt in the water and have hardness and a lot of nitrate you're just begging for kidney dysfunction. Add to that a little toxic material as a trigger and boom, ill fish.
Air flow. Are the tank tops covered? Betta breathe air, ever step outside into the winter and suck three or four lung-fulls of air in THROUGH YOUR NOSE? It can give you a common cold pronto, it can kill your fish by stress.
Dust/fake food. Any bug that enters the tank is fair game for dinner, at one point I had my girls trained not to eat anything but food-food but somewhere they lost that. Ants, spiders and roaches are poisonous to betta in different ways. Betta in the wild eat wild bugs all the time but we're talking about a fish who's species survival is based on a capacity to produce five hundred to eight hundred babies in one go. They will eat just about anything, including bits of metal, dandruff, pet dander, hair, plastic, carbon, silk plants and so-on.
Suspect everything you put in the tank! Silk plants from non aquarium stores will ALWAYS have toxic materials on them and may even not use permanent dyes. Silk plants from aquarium stores MAY have toxic chemicals on them but likely don't have non-permanent dyes.
Quarantine! If you quarantine new fish when you bring them home you should also quarantine plants. If you constantly have to deal with infections and illness in fish you get from a pet store, tell them. If they shrug it off report it to Animal Control. Takes one time for them to learn how important it is to not make the fish they stock sick. If the fish are always sick you can be assured that the plants are sick.
Do you aerate the tank? It might seem unnecessary to do with betta breathing atmospheric air but keeping the water well aerated makes it very very hard for anaerobic bacteria to hurt your fish. The danger from anaerobic bacteria is very simple, they can live anywhere there is energy for them to survive. This means under scales, in gills, under the skin, between organs, inside rocks, inside dirt, inside plastics, in activated carbon, inside the solid magnet of impellers and under whatever amount of gravel you've placed in the tank. Aeration is cheap. Walmart has small tank pumps for under ten dollars, bigalsonline has tetra air pumps that have the same innards for $8.00 right now. You can simply drop an air-stone into the tank on the end of a hose (use longer hose than you need, then cut off short bits when it starts to be loose on the pump or stone) or use some kind of bubbly ornament. There are usually little packs of valves and T-connectors at most aquarium stores, you simply put a T into the line between the pump and stone then put the valve off the T to let out some air to keep the bubbles to a minimum. This breaks the surface AND oxygenates water.
Betta live more than a year in the wild, look at and examine everything you do different from a deep mud puddle in a warm breeze.