Duplarit G -- is it worth the money?

I seem to have learned the basics of keeping the fish alive, but am still
hopeless with aquatic plants,so am considering starting again using Duplarit
G in the substrate. So here are my circumstances--perhaps someone will be
kind enough to advise whether Duplarit will be worth the cost.

Tank is 75 gal freshwater community setup about 4' x 18" x 2', with a 150
Gal/hr trickle filter & a Fluval 4 plus internal filter (260 Gal/hr) which
I run at about 50% flow. It has been established for 9 months.

Lighting is 3 x 30w T8 fluro tubes (Actinic blue, ColorMax and plain white).
I realise lighting is low, but cant do much about it because the hood is
pre-formed plastic to suit the tank (once piece of glass with rounded
corners). Lights on for 10-11 hours daily.

There are about 75 mainly small fish, such as neons, mollies, barbs,
rasboras, dwarf gouramis, corys, loaches, guppies, glass cats, danios etc.
I have, I believe, learned not to overfeed. Although I do get quite a bit
of green algae, but that is probably due to the fact that the tank gets
morning sun for about an hour--but the fish like the sunlight, so I can live
with the algae.

Substrate is aquarium gravel, with several largish pieces of driftwood.

Weekly water change of 20%; test results consistently as follows: pH 7.8,
EC 400, GH 7, KH 5, Nitrate 7, Phosphorous 0.75

The Krib CO2 table tells me that my water is providing virtually nil
available CO2, alas.

Now we come to the poor, sad plants. They are just the usual val, java
fern, sword plants, elodea, heteranthera. They cover about 20% of the
bottom in a more or less sickly, greenish-yellow looking miserable mess.
The val grows ok, but thats about all that does. Ive been trying various
combinations of excel, excel flourish and flourish nitrogen, using them at
the recommended dosage for a couple weeks, then adding or subtracting a bit
here and here for a couple weeks, looking for the right ratios. But, alas,
its still an essentialy dismal failure.

So my plan is to buy 1.2Kg of Duplarit G and start again. But that will
cost about $100 Australian. So I dont want to go to all the trouble and
expense unless there is a reasonable chance it will make a big difference.
What do you folks suggest I do?