I just put one in my 40g tank. It took me a full day of fooling around and positioning to get it right. It is hard to get the flow distributed enough in a small tank that the flow is low enough for horses.
I have the pump set up just under the water surface in the part of the tank where my horses spent the least amount of time in the past. The flow for 2 feet in front of the pump will really push the horses around. The flow in the area where I am puttintg some gorgonians is quite strong, but the horses seem ok with it and the water in the rest of the tank is all moving.
Overall I really like the pump, but a 40 is the smallest seahorse tank I would put one in. I think it would be a great circulation pump for a 50 or 65 gallon tank.
Fred