Plant difficulties

I used to be able to propagate plants in my tank. Not the really
demanding ones, but the duckweed, etc., but now, absolute NOTHING
thrives in my 20 gallon tank. The fish seem happy enough however.

Our water is fairly soft, and I pay attention to the PH, though it
tends to run towards acid... 6.4 when I tested it today. I use an
outside box filter (Eheim) which doesn't give a whole lot of aeration,
though the fish are not gasping. A couple of sprigs of Hygrophila
Polysperma as floaters have just "passed away". I was giving the tank
at least 8 hours of fluorescent light per day, Water changes of about
25% weekly.I was told by a LFS that this plant is pretty hard to
destroy, but for me, no problem!

I have 4 corys, a pair of Siamese algae eaters, 4 neons and 4 cherry
barbs. Down to one solitary male guppy now.

Should I try a plant supplement, get a Gro-Lux tube or what?.
Bulletproof plant suggestions would be appreciated. Wouldn't mind
repopulating with some more livebearers, so some sort of
surface-floating plant to provide a a safe haven for fry would be
preferable.

This aquatic brown thumb needs advice. Thanks.
Blair.