The best replacement for substrate is enzyme.

The best alternatives to substrate are rated as following:
★★★★★  enzyme
★★★★  oxidative phosphorylation
★★★★  gravel
★★★  ligand
★★★  inhibitor
★★  active site
★  sand
★  inducer
★  xposed
★  enzyme concentration
★  reactant
★  reagent
  cofactor
  metabolite
  oxidative
  reactants
  subfloor

Your alternative for substrate is not listed? Suggest!
Speaking about substrate, one probably shouldn't forget about fish in tank.

Other search terms related to substrate and their ratings are:
★★★★★ fish in tank
★★★★★ enzyme
★★★ planted tanks
You have in mind a companion for substrate and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of substrate in all years. Click on graph for more.
substrate
enzyme
oxidative phosphorylation