I agree with Andy. The tank is most definitely still cycling and the ammonia is poisoning your fish.
You put a COLOSSAL amount of fish in there when it wasn't cycled......I'm surprised you have any left
It's normal to add a couple of fish at a time when you're cycling with them.
I also think you need to do a 50% water change and do it again whenever your fish look like they'e suffering. Do it daily for a while if necessary.
It'll prolong the cycle but otherwise you may well lose all your fish.
Don't replace any of the losses or add any more fish until your ammonia and nitrite a steady at 0ppm for a couple of weeks.
I also agree that you should stop adding fertilizer and algae destroyer for now. The ferts will be feeding your algae, and the algae destroyer vastly adding to the stress your fish are under atm.
Because your filter clogged fast and you have so much algae and a film on the water I think you're overfeeding and that will be adding to the ammonia spike.
Have you got a gravel vacuum for the tank? If so....clean the gravel at every water change, make sure you rinse the filter sponge at least once a week with DECHLORINATED water.
You
are dechlorinating the water when you do water changes aren't you?
Feed less. You only need tiny amounts of food...as a rule only what your fish will COMPLETELY eat in 2 mins...and that means nothing left. No flakes left on the floor of the tank. I'd probably feed only every other day until the crisis passes.
Definitely add some bunch plants...Hornwort is great and has algaecidal properties, or Canadian pond weed...anything like that. They don't have to stay long term....but they'll help while you're cycling.
Hopefully everyones advice will help.....but in the worst case senario and all your fish die, don't give up on the hobby.
You need to do a lot of reading up on cycling and take things really slowly. You'd also need to sterilise everything in your tank before you start again and bin the plants. If that happens...get back to us and we'll tell you how to clean your plants so you don't introduce the dreaded black beard algae (which it sound like you might have) and how to introduce fish slowly.
This hobby has a very steep learning curve for all of us so I know how you feel....but it's so worth it if you don't give up!