Honestly, this is what I would do with a tank in this kind of condition.
Take the fish out, place them into a container/bucket (seperate containers if aggressive) with the murky water. Drain the tank to as low as it will go, vacum debres, remove as much dead plant as possible as organic matter lowers ph. I dont recommend almost 100%
water changes but in this case theres just too many problems to fix with a smaller one.
Refill tank with clean dechlorinated water,
test the water from the tap so we know your
tap water levels, let a glass of water stand for 15mins before testing.
Now you are going to
very slowly,
acclimate the fish to the brand-new aquarium water. Like they are brand new fish. Remove some murky water, so just enough covers the fish and then slowly dribble fresh aquarium water into their container very slowly, the fish should be calm, if they aren't you are pouring too fast. Once container is around 70% clean water, you can take the fish out and release it into the aquarium.
You now have to complete the
nitrogen cycle, it will take around a month.
You will need to test the water at least every other day and dont overfeed, making sure
ammonia and
nitrite remain under 1ppm.
Since you bought medication I am going to assume you arent against spending money, so grab some
seachem prime. It's going to protect the fish from ammonia and nitrite until the bacteria colony is big enough to eat it. It will save you having to change the water everyday while the cycle completes. Its expensive but a little goes a long way.
You must dose every 24hours, and if the ammonia or nitrite go above 1ppm, do a water change. Only double dose prime in an emergency. Eventually you will get readings of 0 ammonia, 0 nitrite and 10-20
nitrates, indicating you have a bacterial ecosystem in the tank and are cycled. Then its just weekly water changes to stop nitrates getting too high.
Lastly, depending on your tap water test, if you have a naturally low ph (like me) and low gh/kh, you may have to grab a small amount of dry
coral, an inexpensive and stable way to stop ph dropping. It can be mixed into substrate, put into the filter or just hidden in the aquarium.
Sorry this was so long but I hope atleast some information helped you and you can save the tank.