Early Tank Biofilm
Weeks 1-4 of a new tank: white slimy film on driftwood and decor. This is a mix of bacteria and fungi colonizing new surfaces.
Beneficial — serves as food for shrimp and many fry species. Fades naturally as tank matures.
Diatom (Brown Algae) Phase
Weeks 2-6: brown dusty coating on everything. These are diatoms feeding on silicates from new substrate and tap water.
Normal and temporary. Otocinclus and nerite snails eat them. Fades as silicates deplete (2-3 months).
When Biofilm Is Bad
Persistent slime with rotten smell + fish gasping = bacterial bloom from uneaten food or ammonia spike.
Treatment: stop feeding 2-3 days, do 50% water change, check filter, reduce stock.
Biofilm as Fry Food
Shrimp, fry, and otocinclus feed on biofilm. Don't over-clean a newly cycled tank — let it mature.
Scrubbing surfaces removes beneficial biofilm along with visible matter. Clean only glass you're viewing through.