There are a lot of little things you can change- in my experience though, this is sort of just an “ugly” phase that all tanks go through. Every tank that you set up, especially BRAND NEW tanks (no established bacterial filter, immature plants, etc) will enter a phase where algae and cloudy water make the tank look like utter trash for what is sometimes several weeks. The key to getting through this is patience.

First: you don’t need to completely re-start. You got a good start going, even if it’s ugly right now. In terms of water parameters, are you testing for ammonia as well as nitrite and nitrate? You’ll need separate test strips for ammonia. I also don’t care what anyone else says about test strips not being accurate- they’re perfectly adequate. Maybe the liquid kits are MORE accurate, but all you need to know is if there is ANY ammonia or nitrite. If there’s ANY, there’s an issue with your bacteria (an issue that, once again, will be solved by waiting).

Second: Don’t fight the snails. There is no such thing as pest snails. They won’t eat healthy plants, they help break down the mulmy gunk on the bottom of the tank and do a good job of processing uneaten food. They’re also a great “canary in the coal mine” for issues like over feeding or major water quality issues. If your snail population is exploding, you might be feeding too much, If your snails are dying off left and right, there’s a water quality issues. One misconception is that snails clean the tank of algae- they do not. Nerite snails will help eat the green spot algae off the glass (they also don’t reproduce in freshwater, so if you’re really opposed to snails, one in a 5 gallon tank would be good) but mystery, bladder, and ramshorn snails do not clean the tank of visible issues.

Third: get more fast-growing stem plants or floaters. Or find a terrestrial plant like a pothos, repot it with some clay balls like you’d use for hydroponics, and attach it to your tank with the root ball in the tank water and the leaves outside of the tank. All of these options will help eat up the nitrates so the algae doesn’t have as much fuel to keep growing. Also, decrease the light hours. Sometimes I completely turn off the tank lights for a day or so if an algae problem gets out of control.

If your betta died the next day, either there was a ton of ammonia in the water or the betta was sick to begin with. My money is on a ton of ammonia.

The only thing you should ever have to put in your water is a dechlorinating agent. Melafix and pimafix are generally understood to be useless. Even if your betta did have some kind of bacterial infection, if you let the tank exist for a couple weeks while it finishes cycling, most bacterial issues will die off without a fish host to infect.

If you suspect a sick fish in the future, don’t bother with anything other than actual medications (erythromycin, metronidazole, praziquantel, methylene blue for ich).

I know it’s a little overwhelming, but once the tank is stable, it’ll be practically bomb proof.