powerhead

I made an observation that when I swapped the impellers in my 1400gph powerhead and 550 powerhead, the output of the 550 increased and the 1400 decreased. The impellers and base units appear to be the same size. I would've never thought the impeller made the difference instead of the base...

How often do you see used VorTechs available? I would like to use MP10's on both my 5.5 gallon and the 65 gallon, so I would be looking for three of them total.. But I know I wouldn't be able to afford to buy them new.. Even used I'll have to do one at a time. My question is how often do you see...

So I just got my first powerhead (a Koralia Evolution 550), and I'm wondering how many hours a day I should run it. Should I put it on a timer and only run it during the day, or when the lights are on. Or should I just leave it running 24/7. Let me know what you think. Not sure what to do...