Do you have a test kit? If so test for ammonia, nitrite, and nitrate and post the results. I'd bet that the nitrate level is high and that is what is feeding the algae and allowing it to take over. The solution to that is more frequent, larger water changes. Most of us change at least 25% of the water every week to keep the nitrate level low. On the schedule you've mentioned, not enough of the nitrate (which is the end result of the cycle as ammonia from fish waste is converted first to nitrite and then nitrate) is being removed and is available as fertiliser to feed algae growth.
I suspect the fish had an internal bacterial infection (hence the stringy white poo), and then the presence of both ammonia and nitrite in the tank water over the time he was in there was enough to finish him off. Ammonia and nitrite both need to be 0 all of the time.
You may be overstocked, definitely were with the clown loaches, who by the way were small for six years old and haven't lived long as the tank is way too small for them They grow to about 30cm long and should live 20+ years in the right size tank. How many danios and tetras have you and what species is the algae-eater? How often do yo change any water and how much each time? The other thing that makes green water worse is if the tank gets a lot of daylight from a nearby window and/or tank lights are on for too long.
It looks to me as though nitrite is at 5ppm, which means, in reality, it may be considerably higher as that's the limit of the test. Nitrate in the tank is higher than the tap water so some nitrite conversion is happening. This will continue, slowly at first but then more rapidly as the colony of nitrite eating bacteria expands. You may not see a drop in nitrite on the test as it will take a while to get down below 5ppm, but nitrate should continue rising to give an indication that things are proceeding. I would estimate you have about a week to go.
Re: New keeper : Q : When do I use all these products in the tank?
The water conditioner removes chlorine/chloramine and neutralises heavy metals. It should be used on the first fill, at every water change and whenever you top up. The biological enhancer adds beneficial bacteria. You can add a dose to help your fishless cycle to establish, at the same time as you add your first dose of ammonia (see fishless cycling article here: https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm ). The biological cleaner is a pointless chemical and I would not use it. The final bottle seems to be plant food - use it as directed if you have plants.