Have you considered regenerating your GFO to save on the cost? Very easy to do. I blow through GFO because I have a high bioload. I stock heavy and feed heavy, and I couldn’t maintain proper levels without GFO regeneration (or at least my wallet couldn’t). I use a BRS style cartridge reactor and keep the GFO in the reactor and simply take one out, swap a regenerated one in, and run the used one through the same style reactor in a bucket with sodium hydroxide for 3 days. A 5-10 gallon rinse of RODI when finished and you’re good to do. Here is the article I use with the specifics:
Aquarium Chemistry: Regeneration of Granular Ferric Oxide Media with Sodium Hydroxide
For aquarists who keep non-photosynthetic corals that require massive feeding, have large tanks, or a dense fish population the costs of regular GFO replacement can be significant. With a few simple tools and techniques we can easily regenerate GFO so that it can be reused several times over.
reefs.com