I made some Ferrous Gluconate solution to test on marine macroalgae. It seems to work prety well and has really low toxicity to marine life. Also it didn't affect the growth of microalgae too much. If you want to grow your macros you can buy it commercially sold as freshwater plant iron supplements. such as this Seachem Flourish.
http://www.seachem.com/Products/prod...urishIron.html