Old Tank Syndrome is a slow-rolling crisis common in tanks running 2+ years without consistent water changes. Mechanism: nitrate accumulates over months. KH + GH drift down (depleted by fish + plants). Trace minerals run out. pH slowly drops. Eventually fish stop reproducing, gain disease susceptibility, and start dying. Symptoms: long-term residents fade, color loss, lethargy, refusing food, dropsy in older fish, sudden cycle issues. Recovery: 25% water change, retest, then 25% weekly for 4-6 weeks while monitoring. NEVER do a 75% change immediately - the parameter swing kills more fish than the OTS. Prevention: consistent 25-30% weekly water changes. Test KH + GH monthly. Replace activated carbon + media on schedule.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
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