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Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 1:48 pm
by talonbro
So I've got this 20 gallon tank that's been running for months, but I had to move my fish out. If I leave it empty with just the filter running, will it stay cycled? Or is this a lost cause?
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 2:27 pm
by stormyok
It depends on how long it's empty. The bacteria need a food source, so if there's no ammonia, they'll start dying off. You could add a pinch of fish food every few days to keep them alive. Worked for me when I had to quarantine my Betta.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 2:41 pm
by leopard
stormyok wrote: āTue Jul 15, 2025 2:27 pm
It depends on how long it's empty. The bacteria need a food source, so if there's no ammonia, they'll start dying off. You could add a pinch of fish food every few days to keep them alive. Worked for me when I had to quarantine my Betta.
Yep, this is the way. I'd also check your params weekly to be safe. If nitrites spike, you'll know the cycle's crashing.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 3:16 pm
by Oceanix9
I've kept a tank cycled for weeks by dosing pure ammonia. It's more precise than fish food, but you gotta be careful with the amount. Like, a drop or two in a 20 gallon is plenty.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 3:40 pm
by Kairos9
How long are you planning to leave it empty? If it's just a week or two, the bacteria might hang on, but longer than that and you're risking a full restart. I'd second the ammonia dosing if you're gonna be awhile.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 4:38 pm
by talonbro
Thanks, everyone. Gonna try the fish food method since I've got some lying around. Appreciate the tips.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 4:43 pm
by carla_s
Just don't overdo the food, or you'll end up with a nasty mess. Been there, done that. Took forever to clean the gunk out of my substrate.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 5:27 pm
by velvet8
My LFS guy told me to use a raw shrimp if I was gonna be gone a while. Let it rot in there, then pull it out. Sketchy, but it worked.
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 6:09 pm
by novakid
shrimp method sounds wild but lowkey makes sense. bacteria gotta eat something, right?
Re: Will an empty tank stay cycled?
Posted: Tue Jul 15, 2025 7:03 pm
by starlord
velvet8 wrote: āTue Jul 15, 2025 5:27 pm
My LFS guy told me to use a raw shrimp if I was gonna be gone a while. Let it rot in there, then pull it out. Sketchy, but it worked.
That'sā¦ creative. I'd worry about the smell, though. Ammonia dosing seems cleaner.