Is the cloudiness white or green? White is bacterial bloom, green is algal bloom, microscopic algae floating in the water.
I asked about timescale, cycling etc as white cloudiness is usually a bacterial bloom which is common in new tanks, but these are not the same bacteria we need to grow in the filter. They are harmless to fish, but can cause problems as they use a lot of the oxygen in the water. Doing lots of water changes will help to oxygenate the water temporarily as adding water churns up what's already in the tank, but doesn't do much for the cloudiness as these bacteria multiply faster than anything we can do to remove them. But as long as your filter outflow ripples the water surface there should be enough gas exchange going on for both fish and bloom bacteria. And going cloudy after 7 weeks is a bit unusual unless you had plant problems just before then as dead or dying plants matter is organic and will feed the bloom bacteria.
Algal blooms are caused by ammonia and other plant nutrients in the water. The usual remedy is to completely wrap the tank with something that blocks out all the light and to leave it there for 3 days. This deprives the algae of light which kills them.
Can I ask what type of testers you are using, strips or liquids? It is strange that you have a nitrate of zero, though not shaking one of the bottles in a liquid tester does give false results. There is usually nitrate in our tapwater, varying from very low up to as much as 40ppm, and the filter bacteria also make it from the ammonia excreted by the fish. Strip testers that you dip in the water are known to be inaccurate.
Gasping at the top of the tank is lack of oxygen - but it is not necessarily due to not enough oxygen in the water. Ammonia burns the fish's gills making it hard for them to absorb oxygen, and nitrite binds to their red blood cells stopping them carrying oxygen. To the fish both feel like they can't get enough oxygen so they gasp at the surface.
Despite your test results, I do wonder if you had some nitrite and/or nitrate as both of these are acidic and coupled with your low KH could explain your drop in pH and the fish deaths. The Equilibrium you have been using sounds like it is the kind of remineralisation salts that you use with RO water (I just looked at Seachem's website). It should be OK. I think.
I do need to make a comment on your fish. 25 litres is a small tank - I have a 26 litre tank for my betta. Small tanks are hard to keep the water stable. If something goes wrong, it goes wrong fast. You had a lot of fish for the size of tank. It doesn't matter what the size is when you buy the fish, it's the adult size you need to take into account.
Otocinclus are shoaling fish, you need at least 6 of them but a 25 litre tank won't be able to produce enough algae for 6 otos. They should really be added to a mature tank that has had time to grow algae for them to eat.
Guppies are OK provided they are all males. If you have any females you will soon be overpopulated with fry.
What were the red tetras? If they were ember tetras, fine, though you need at least 6 of them. Any other red tetra will grow too big for 25 litres.
Julii cory. All cories should be in a shoal of at least 6 of the same species. There are only three, maybe four species suitable for a tank your size and I'm afraid juliis aren't one of them. Yours would also have probably been trilineatus as that's what nearly all the cories labelled as julii actually are.
A snail is fine.
Do you have any fish at the moment? Are the fish you listed what you have, or what you had? If it's what you have, could I suggest you rehome the cory and find out what the red tetras are.
I'm not very good with plants but I think that you don't need the Flourish Excel unless you have high lighting levels, something that small tanks don't usually have. Hopefully one of the plant experts can help you with them.