Mine can easily suck up a fish small enough to fit into the vac tube. My vacuum tube is about 2" wide 18" long. It has a boll valve right on top where the hose connects. Normally I stuff the truce into the gravel and clamp it to the tank wall. This way no fish can enter the vacuum end.
When I am actually vacuuming with it, I watch carefully and keep ready to close the valve. So far I have only got one silly Swordtail she swam right into the vacuum and up the hose. Had to close the valve to stop flow, then turn water on to fill tank and open valve on the vac and she came right back out into the tank. No worse for ware.
It is something you have to watch for when vacuuming. I don't vacuum very often and when I do only a small portion of the bottom at a time. To much can cause algae blooms and other issues is what a discovered years back.
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