Not too much. I use it in some 10G tanks, with moderate light. To maximize efficiency I removed the media basket in an AquaClear Mini (20), and use two sponges of the following model (30) stuffed snuggly. This keeps some bubbles trapped beneath them in the flow, some of them going through the sponge pores. I also fill the bottom of the filter with ceramic rings. Good dissolving chamber.
The unwanted surface agitation can be minimized. There's a certain water level at which the output disrupts less the surface. I also keep the flow reduced all the time, because the filter is still too much for a 10G.
I once withdrew a pH meter from the lab for a few days. Measures gave between 10-20ppm.
Plants grow well with 2x13W two pin CF bulbs with reflectors. However, I have to say, I once used 3x13w, with two of the bulbs on one side of the tank, and things didn't go so well, algae on the glass and on plants in the brighter side. I wouldn't say this is a good method for high light, but the previous config has worked very well on four tanks identically equipped. One more note, it seemed to work better in the 15G hex, assumably due to the lower surface-to-volume ratio.
Cheers, and welcome.
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