Iron gluconate ought to be available as well, adding Fe to the deeper substrates that have some organic matter will reduce the Fe3+ to Fe2+ naturally via bacteria and low O2.

The chelator is rather weak with gluconate and is often what's used in Vitamins and in the SeaChem Fe. This means it's very easy to break the bond for the plant, but it also means it will not last in the environment long also.

DTPH last longer and is a nice middle ground.
Iron filings, iron "dust", nails etc have been added to substrates in the past and seem to work.

Regards,
Tom Barr