My Filter (Penguin 170) doesn't provide enough current in my 20 gallon high planted tank. If I lower the water level I lose CO2 so I was thinking about adding a small powerhead. Any suggestions? I have some cories, and 3 neon tetras and 3 zebra danios in there right now but will either add more neons or remove them and get another schooling fish. I don't want a lot of current but the water in the tank is barely moving and debris is setteling on the bottom.