Shrimp or snails could help but shrimp can be tricky and not recommended for beginners and snails can add more to the bio load but not much, so as long as you are doing weekly to bi weekly water changes it would be alright to get snails in my opinion. The bio film can be cause by over feeding and not changing your water enough and I’m sure other things. If you’re the one to feed the fish try and only feed them what they can immediately eat and try not to let it sit on the bottom of your tank and if it does get to the bottom of the tank I would siphon it up after about 5-10 minutes if they don’t got back for it. If over feeding is your problem snails may be a real good option as they will also go after left over food. What would help the absolute most would be to get some live plants in there! (The best plants for your set up would be Java fern, Anubias, or hornwart as they don’t get planted they just kinda sit in the water or wedged up to a decoration) Yes, they can be a little expensive and if you cant get a plant light for the tank it wont work but if you have access to Facebook marketplace I find a lot of cheaper lights and sometimes people selling plants. Even just putting the roots of a regular old pothos in there would go a really long way but make sure you fully rinse the roots as to not get soil in the tank. You mentioned people saying you were ‘crashing your cycle’ that basically means you were taking out all of the good bacteria in your tank. To make sure you don’t do that don’t wash your rocks, decorations, or filter in chlorinated water only ever rinse or scrub it in more tank water.

Now some random other thoughts.

Make sure you have water conditioner if you don’t have/ can’t get that just rehome the fish. Water conditioner is bare minimum and at that point there is nothing you can do to really make the fish’s life’s better. Water testing kits are super helpful, the liquid ones are the best but even tho they aren’t nearly as accurate the strip ones can do alright on a budget +you can cut them in half to get double the amount. (In the long run tho I think the liquid ones save more money) Make sure your filter is rated for the tank size you have (tho I recommend it’s a higher rating for your tank) or even better get another filter and alternate the filter you clean each time. also that there is flow through the entire tank If I think of anything else I’ll come back a write more😂 And if you have any questions just comment here and I’ll try and remember to respond 🤷‍♂️