How long will my bacteria be ok in a tank with no fish?

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Wanda you have a light hand with the ammonia. People who aren't as cautious as you sometimes find they have a nitrite spike after dosing NH3 in an established but temporarily fishfree system.

And some folks think that the nitrite-eating strains of nitrifying bacteria are inherently sensitive to ammonia. Part of why they don't get a grip til the NH3 is being all used up by the ammonia-eating bacteria. I can't vouch for this. But sometimes "re-seeding" after the NH4 is at 0 will reduce the nitrite spike and speed the cycling.

Bacteria live at the edge of starvation normally. Any extra nutrient source and they rapidly increase to assimilate it. The population restabilizes at the edge of starvation again.

An established system, with undisturbed rocks, gravel, filtration, maybe even plants, is pretty sturdy. But I may be judging merely from my own setups. I've never run a barebottom isolation tank. Or even an aquarium without plants-- at least not since about 1960...