very brittle and glassy leaves - the slimy green

hi,

i'm new to these forums, and to aquatic plants in general. i hope this is the appropriate placement of this thread. i figure this issue, while overwhelming to me, is probably something you experienced people have dealt with long ago.

first of all here are my tank parameters:

substrate - seachem's flourite (several years old)

temp - 80-83F
ammonia - 0ppm
nitrites - 0ppm
(the tank finally cycled!!! )
nitrates - 10ppm
ph - 6.8-7 (slowly drops to 6.4)
gH - >300ppm
kH - 40ppm
lighting - (2 x powerglo 24w t5 bulbs, 1 x 14W unknown generic aquarium bulb > 10,000k)

i don't have any CO2 generation.

i have... rather HAD a really thick/lush planted aquarium initially. other than the amazon swords, and some underwater ferns, and one of those onion plants, i don't know of the other plants. (i'll take pictures in a bit.)

unfortunately, those plants don't survive too long, or in the case of the plants with large broad leaves, the large leaves become transparent and yellow, and the new leaves are thin and long. even the new leaves are glassy. with another plant with thick leaves, the roots rot away, never grow back, and the new leaves are curled.

while the smaller amazon swords do well with their roots, the larger ones, along with the other plants do really poorly. i figured the flourite would cause the roots to flourish, but it wasn't the case. as a result, i went out and bought seachem's substrate/gravel fertilizer tabs. i planted about 5 in the area covered by a 29 gallon tank.

oddly enough, i have very healthy algae growth. i don't know what kind of alage, but it's green and extremely slimy. apparently, the angefish love the stuff, but they don't eat nearly enough to keep things clean. the stuff grows everywhere, on the glass, on the filter cascade point (where it thrives), and all over the leaves. it's a pain to clean off of the leaves, and when i do, the leaves tend to break.

in addition to this slimy stuff, i have these really circular, hard, dark green spots all over the glass. it takes much more effort to scape these clean. and when i do, even more are back after a few days.

from what i've found, this is a symptom of a lack of iron; however, i thought this substrate even though it's old, in combination with those tabs, would give the plants the iron.

and ideas how i can solve both of these problems?