What Is Causing My Water Sprite To Turn Brown And Die?

[bluesky2111]

This is the battle I've been fighting for 2 years. I still have no idea why my water sprite keep turning brown and dying (along with other plants showing different kind of deficiencies, but I'll leave it for now). They will keep shooting new leaves, but as soon as they become big enough they turn to brown and melt away. What tf is this? It's weird that there are people who can't stop this plant from growing while others can't keep them alive.

My tank: 75gal. No CO2.
Fert: Root tabs, Flourish, Excel, Iron, Potassium
Light: Finnex Planted+ 247 / 7 hours a day
PH: 8
GH: 200ppm
KH: 100ppm
Nitrate: 20-40ppm
Nitrite, ammonia: 0

 

[Roger121]

looks like black beard algae, the reason maybe is the fertilizers in a tank without a not so good light coz I use the half to the products u use in my high tech.
the exel if im not wrong is glutaraldehyde and kill the brown and green algae but don't work against the black