Let us not forget about Prolfile/Turface/Schultz's aquatic plant soil.
The weight issue can be dealt with by adding 25-50% sand for added
weight and ease of planting etc.

"Excess"(?) Iron has not caused algae in any tank I have critically
tested it on.
Algae needs such a small amount and so do plants for that matter
before it becomes limiting you simply do not possess a test kit that
even comes close.
Most researchers do not have the equipment to limit and test for Fe
limitation for FW algae. Some Marine specialist do.

You will not be able to limit an alga by Fe limitation because you
cannot tell if there's enough Fe to become truly limiting or not.
A plant leaks and dies/leaf is dropped and decompsed etc and leaks out
a tiny amount of Fe and you would not even get a chance to test it
before it was assimilated etc.

A similar thing happens with NH4 but at higher concentrations.

Regards,
Tom Barr