How to drill anti-siphon holes?


New member
I bought my tank used a few years ago and have always had to deal with the siphoning issue when my return pump is off. The ball tubing on the return outlets don't have any kind of anti-siphon holes so I end up draining like 3" from the DT into the sump. Thankfully my sump is sized such that it can handle this, barely.

I would like to actually fix this issue so as not to have this ongoing worry in the future. Maybe this is a dumb question, but how exactly do I go about doing this? I assume that I'm going to need to do this during a water change when I have the water level down below the returns, but how exactly do I drill this? Do I worry about the type of bit due to copper or other metals coming into contact with the water? I assume that I drill it on the highest portion of the return line? I also assume that the smallest diameter drill bit I have will be sufficient? What other questions should I ask that I'm missing?

Finally, this is probably another dumb question, but... If I have a hole in the return line, won't water come shooting out of that hole when the return pump is on?


New member
I just use whatever small bit I have on hand and a drill. As far as water coming out.... Yes it will. I like my holes drilled below the water level when the tank is running but above the water lvl when the return is stopped.


New member
I'm just afraid of shooting water up onto the lights by drilling a hole in the top of the return tubing. Maybe if I drill the hole slightly angled opposite the direction of the water flow that will help minimize the pressure of water coming out of the hole?


New member
There is really no need for an anti-siphon hole when your sump is sized properly as yours seems to be..
Its only an issue when the sump is not sized properly.

If you still want to then just drill 2 holes just slightly under the "normal" water level when the tank is running normally.. Don't get the drill wet (and use a cordless model)
Water WILL come out those holes but its underwater and should not cause any issues.

Or just mark normal water level and shut off power then drill when the water level has gone down..

Its not rocket science.. Don't worry.. Drill on


New member
Careful... Even though a correctly sized sump is important, there may be a very great need for a anti-syphon depending on the plumbing and how far down into the water your return sits. My main return, for example, feeds back through the overflow and then into the tank 1/3 up from the bottom of the tank. if I didn't do any anti-syphon, I'd have about 80 gallons of water on the floor during a power outage.

However, the mechanics of drilling the hole is not that big a deal. I tried to drill it the direction I wanted water to come out (at an angle - as best I could downward into the water instead of up towards the lights).


Grizzled & Cynical
I never liked the salt spray from the anti-siphon hole, so always have stuck a bit of airline hose into it so I can better direct the water that shoots out.


aka John K
This is the kind of thing that's best planned right in the first place. Anti Siphon holes can and do get clogged up, so if you are counting on them you are likely to have a problem (much like using check valves to address the same problem).

For the OP, it sounds like your sump is sized just about right and you are good to go. Adding a secondary safety measure that may not be reliable would not improve on a design that's already good, but it might create a false sense of security that could lead to problems down the road (like if more equipment was added to the sump making it's currently adequate volume become inadequate).


Active member
I have returns close to the surface to create surface agitation and to not have to rely on anti siphon holes that could clog. My sump is sized so that I can pull the standpipes and completely drain the overflow box without fear of overflowing my sump.

Sounds like you are set up well too. I'd leave it like it is or maybe turn up the return toward the surface a little.


New member
Mark the top of the water on the return pipe then turn off the rturn pump and drill a hole about quarter inch below your water mark.

Here is a pic of my anti-siphon hole with the return on. Note it is below the water surface

Pic with return off



New member
From looking at the pictures, I would drill a hole right above the dark circle of the vertical pipe.

The dark circle on the vertical pipe is the anti-siphon hole. The hole is 1/4 inch below the water level so that it doesn't spray above the water line with the pump on. If you drill it too high you will get too much spray.


New member
How are your returns set up? If you have loc line, I have seen people make an arch just above the water line, and drill a small hole facing down so when the tank level drops under the top part of the hose, it breaks the siphon. Plus, it's just one more point of random flow!