For everything to simply wither and melt it's more drastic AND simple, something is being missed beyond what's been the focus so far.
Do check your water. If you do not have a test kit, take a sample to any fish store (PetSmart/Petco/LFS) to be tested within 30 mins (gH, kH, pH, etc). Find your water district's annual water report for more information. The more we know, the less guesswork.
Now, I have exactly the same tank and it's been a pain for ~7 years. After doing almost everything I could think of, I am still not happy with it: I cannot grow vals, crypts, water wisteria and a lot of other plants. The only plants that seem to survive are Java Fern, H. tenellus, Creeping Charlie, L. sessiflora, and Ludwidgia repens (just barely). And I grow some pretty demanding plants in my other tanks. And this guy sits right next to my computer, so, I'm seeing it day in and day out.
IMHO, the biggest problem with that tank is circulation: I just cannot get it right. I started with Eheim 2026 with the spray bar in all locations you can think of. I'm currently on Eheim 2075 with Koralia 425 and the tank is getting better. I will replace the Koralia with another canister filter.
The tank seems to be at its peak when it's heavily over-populated and over-planted with no co2 and LeafZone ~ once per week and a bubble wand at night. The more I 'over-think' this tank the worse it gets. I run that tank with no co2 for ~ 5 years and now with co2 for 2 years. The light has not changed: it's Coralife 2 x 65w CFL. Substrate is Flourite. Lighting period is 12 hrs. I currently dose EI-light as in all my other tanks. About 6 months ago I get fed up and just let the tank be: no water changes, no trimming, no cleaning for over 3 months. I was just topping off water and continued to dose ferts. The tank thrived, the fish were the happiest ever, the plants covered 150% of the tank and 0 of any algae.
The reason for the long-winded post is a) you are not alone b) that tank's shape is PITA. Sorry if this is of little help. Let's get your water numbers and go from there.
The first picture is the tank over-planted, no co2, no dosing, except LeafZone.
The tank 2 month ago with co2 and EI. I liked the old tank better