Help With Deficiency (stem Plants)

Hello,

I am trying to identify a nutrient deficiency with my stem plants. Attached is a picture of my water wisteria (upper right) and ludwigia (lower left). Both plants are able to show growth, but after about a week the leaves will get very dark lines, almost like they have been slashed or injured (it's the only description I can think of). The issue is much more prevalent on the Wisteria, but you can see the fine lines developing on the ludwiga. Once the lines develop, the leaf begins to fold over and I start to see algae develop on the leafs and stems, they die off a short time after. My anacharis also has issues, but mainly just general browning/dying of anything over a week old. Basically, any new growth on a stem plant lasts about a 1 - 2 weeks, dies and is attacked by algae.

Another forum suggested I try adding potassium, but after three weeks I am still seeing the issue on new growth. Overall, I am losing the battle and have seen much of my stem plants die off. I also am experiencing a minor outbreak of BBA and moderate outbreak of GSA (mainly on my glass on Anubias). I think the BBA is caused by the continued plant decay. I am not really sure what is causing the GSA.

My tank stats are as follows:

Size: 5 Gal
Age: two years, fully cycled
Water: 50% Tap (treated with Prime) & 50% Distilled
Substrate: Generic Gravel
Ammonia: 0
Nitrite: 0
Nitrate: 40 - 60
PH: 7.5
TDS: 250
GH: 6
KH: 5
Light: Finnex 24/7, which I run on high for six hours, medium for three and a low blue for three at night
C02: I used to use excel, but switched to compressed a little over a week ago to try to combat the algae.
Fert: Nilocg Thrive (1ML 1x a week, but I will now do 2x with the C02). Seachem Potassium (1ML 2x week)
Temp: 75 - 80F
Water changes: 50% weekly, sometimes 2x if the plant decay / algae is bad.

Stock:
2x Neon tetras
3x Harlequin Rasboras
1x Chinese Algae Eater

Thank you for any help, I am out of ideas