Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Start with about a dozen holes, 3/16" drill bit. If this is too restrictive, if the spray is more like a jet (it probably will be) you have a couple of options:
a) Enlarge each hole. Spray will be gentler out of each hole, less chance of crud blocking the holes. Yes, this is after the filter, but still, stuff can get through.
b) Add more holes. but there is still the possibility of any crud blocking the holes, these are small holes.
c) Connect the holes you have with slots that are parallel to the pipe. This will create a more sheeting effect, especially if you can sand the edges for a smooth flow. I would probably not connect ALL the holes all the way across, but connect them in pairs, so there is a slot, then pipe, then a slot, then pipe... If this is not quite gentle enough then make more slots. The thinner the cuts the more forceful the water will come out, but the more slots there are the gentler the flow. Again there is the risk that a really thin slot will collect a bit of debris.
d) Turn each hole into a slot at right angles to the pipe. Same comment as c). Small slots = more force and might collect some debris.