We use these in our 20-30 gallon tanks. One is sufficient to provide enough flow to circulate the tank and keep our anenome's happy. We have been using the same ones for 2 years now and they are still running like champs. The only negative point of them is that after a year of unplugging and plugging back in constantly (every time fry hatch the pumps are turned off and back on and sometimes for water changes) the flow reverses when plugged back in. To remedy it, you just unplug and plug back in and the flow reverses back to normal. This only occurs from excessive unplugging and should not be an issue if you only unplug occasionally. For the price, its not an issue for us as we are saving tons of money. If you want the top of the line expensive equipment this powerhead may not be for you, but if you are budget savvy and just want a decent powerhead at a great price than look no further.
Tip: Place the powerhead on one side of the tank and point downwards and towards the glass so that the current bounces off the glass (Don't point too far downwards or it will stir up your sand). If you have two of them, place one on each side of the tank and face them downwards towards each other so that the flow currents meet and spread throughout the tank.