Many guppy tanks crash after someone “does the right thing” and replaces filter media. The problem is that most aquarium filters are not just mechanical devices — they are biological homes. The bacteria that keep ammonia and nitrite at zero live on surfaces inside the filter. When you replace or over-clean media, you remove the colony and the tank loses its safety net.
The biggest myths
- “Dirty media means it’s not working”: often it means it’s catching waste and hosting bacteria.
- “Replace cartridges monthly”: this is a sales habit, not a fish health habit.
- “Rinse under tap water”: chlorine can kill your colony fast.
What to clean (and how)
Cleaning is fine when done gently. Swish sponges or media in a bucket of old tank water during a water change. You’re removing the thick sludge that blocks flow while keeping most bacteria intact.
What to never do suddenly
- don’t replace all media at once
- don’t deep-clean the filter and do a huge water change on the same day
- don’t “upgrade” filters without seeding new media first
If you must replace something
Replace in stages. Keep old media running alongside new media for a few weeks so bacteria can migrate. That one habit prevents most “mystery ammonia spikes”.
Filters keep guppies alive by keeping waste invisible. Respect the biology, clean gently, and your tank becomes far more stable and forgiving.
