Some suggestions for such a large tank (pretty much what has been state above). I would try other options before giving up so quickly, this is more so a learning experience than a road to perfection.
How old was your tank?
Light setup(distance, length, bulb type)?
Water alkanity and hardness?
1) UV sterilizer (18W?)
2) Soak & Rinse substrate.
3) Inject ferts into substrate instead of water column.
4) Circular or some sort of consistent pwrhead circulation and aeration(maintaining redox is critically dependent on your tank's alkanity/hardness)
5) Depending on your tank setup, did you try a riparium setup or floaters to absorb light/nitrates.