I have never had a sump so just some suggestions.

If you decide to use a sump I don't think you would need to change anything or drill additional holes in the back of the tank, just use the existing holes for drain to sump and return to tank with the heater in the sump.

I can't help with specific details but what size are the holes in the glass?

If you decide to use a canister filter, use the existing holes for intake and output just as the previous owner did.

Do you have a canister filter already and if so, which brand/model?

I think there could be other options if the back glass is not tempered and you want to drill it for either additional filtration or heating.

 

If the bottom is already drilled can it be tempered again? I thought the bottom the one glass part that may not be tempered to allow drilling. Is there a way to visually check?

 

Got polarized sunglasses? Look at the back window of a car with them and you'll see a pattern on the glass. I've heard it's the same with the tank.

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its 1 on each end? mine is 2 on each end. but its a 180, normally the return and the drain on on the same side. I actually have 6 overflow kits for 24" high tanks i never used because i did the herbie method if you get me a picture i can prob show you how to plumb it. if you look at my build log you will see my plumbing

 

SO... a bit of a different take on the whole thing. You could use one as an intake for your canister filter, with the return over the side. Use the second one for a "closed loop" style setup with an external pump and you can plump an inline heater in there, and have the return over the side (this option is likely cheaper than buying a second canister filter to do this with) you get the added bonus of getting increased flow in the tank if you feel it's needed

 

Thanks all for the input. Been trying to figure this out for a while now.

SO... a bit of a different take on the whole thing. You could use one as an intake for your canister filter, with the return over the side. Use the second one for a "closed loop" style setup with an external pump and you can plump an inline heater in there, and have the return over the side (this option is likely cheaper than buying a second canister filter to do this with) you get the added bonus of getting increased flow in the tank if you feel it's needed

That is an interesting idea. Would definitely be the least amount of work/modification to the set-up.

Do NOT put the intake 4" above the bottom, this is asking for a LOT of water to flood and is a VERY bad design. I suggest you silicone over those bottom holes with a piece of glass and not use them. If you want a sump, drill the side of your tank and either use a coast to coast style overflow or a simple internal surface skimmer box with either of these two designs your plumbing will attach to the side of the tank via a bulkhead.
FYI: The bottom cannot be re-drilled once it is tempered. And it probably has been tempered once those factory holes were drilled.

Completely agree, seems like a disaster waiting to happen. Wouldn't even consider it for a sump design, although if you think about it any canister system has the same flaw. If the intake hose becomes disconnected the tank will drain to the level of the intake strainer.

I would use your bottom holes for intake to a sump, and just return over top of the tank. An auto water top-off system in your sump is nice if your worried about water evaporation. Sumps are really nice for hiding equipment, filtering your water, and adding top-off water back in.

I didn't touch on this subject in my OP, but the main issue I'm having with designing the plumbing is how to incorporate a main drain + overflow/emergency drain (ie Herbie-style) when the pipes would be in opposite corners of the tank. I'm not experienced with overflow designs, but I've done a lot of research on the subject. Seems to me like a weir is almost mandatory to keep the system tuned properly, but I don't think having separate weirs for the main drain and emergency drain is possible to keep properly balanced. Only way I have figured to use the bottom holes for the overflow system is to construct a weir in each corner and connect them via a coast-to-coast along the back wall:

Seems like a convoluted solution and would require some major custom fabrication. Also rapidly diminishes the aesthetics of the tank overall.

I see three options.
1) Silicone glass over holes, run more desirable canister with inline equipment. Very easy, very effective.
2) Plump canister to bulkheads in existing holes. You can use hard PVC that corresponds to the bulkhead size and them a barb hose fitting to transition to the appropriate tubing for your canister of choice. Very easy, very effective.
3) Silicone glass over holes, drill back/side, install a bean animal overflow and use a sump.
I suggest posting pictures.

Option 2 would definitely be the most simple solution. For some reason I hadn't considered just adding a reducer below the bulkheads to keep using my existing canisters.
Ultimately I think a sump provides the best long-term solution. I may just have to bite the bullet and drill a few holes to make it work. I'm thinking something like this is probably the best bet for functionality/safety while keeping visual impact to a minimum:
Eshopps Eclipse Slim Overflow Systems | Wet/Dry Filters | DrsFosterSmith.com
I just need to conjure up the courage to put the drill to that 1/2" glass and pray that it all goes smoothly! If I go this route, which right now feels like the best option, I would then use the bottom bulkheads for a split return.

 

I would use your bottom holes for intake to a sump, and just return over top of the tank. An auto water top-off system in your sump is nice if your worried about water evaporation. Sumps are really nice for hiding equipment, filtering your water, and adding top-off water back in.

 

I have a 75 gallon with corner overflow and a drilled bottom as well. I'm using an FX6 canister with it. It was very easy to plumb and get going and filter maintenance is a breeze. My tank is also an open top and I'm adding 1 gallon of water a day to compensate what has evaporated. I'm looking at an auto top-off system now. So just my experience to help you make a decision. I put my heater by my overflow so you don't see it, but once I get an auto top-off system I will place it in the corner overflow so nothing will be in the tank.

 

Do NOT put the intake 4" above the bottom, this is asking for a LOT of water to flood and is a VERY bad design. I suggest you silicone over those bottom holes with a piece of glass and not use them. If you want a sump, drill the side of your tank and either use a coast to coast style overflow or a simple internal surface skimmer box with either of these two designs your plumbing will attach to the side of the tank via a bulkhead.

FYI: The bottom cannot be re-drilled once it is tempered. And it probably has been tempered once those factory holes were drilled.

Bump:

I don't think you can put the ATO sensors in the overflow compartment. That compartment has a consistent level of water and then it runs dry. You would need to put the sensors in either a sump or on the side of the tank. Check out the JBJ ATO it is very reliable and the workhorse of the industry. It uses 2 floats that can be set up in different configurations.

 

I see three options.

1) Silicone glass over holes, run more desirable canister with inline equipment. Very easy, very effective.

2) Plump canister to bulkheads in existing holes. You can use hard PVC that corresponds to the bulkhead size and them a barb hose fitting to transition to the appropriate tubing for your canister of choice. Very easy, very effective.

3) Silicone glass over holes, drill back/side, install a bean animal overflow and use a sump.

I suggest posting pictures.

 

I would make your own weir and go with a coast to coast if you are drilling the back of the tank. You can use the bottom holes for your returns, but then you have a ton of extra plumbing in the tank to bring the returns to the surface. Remember, when you turn the pump off, the return will drain water all the way down to it as well. That is why many people run returns over the back of the tank and have a small hole, a siphon break, just below the waters surface.

 

Drilling is VERY easy I have done it. Just let the weight of the drill do the work and make sure to keep the area wet. Easiest way is to put the tank on it's side and build up a coffer dam using plumbers putty.

Also: A canister filter is a closed system. If the motor stops there will be no flood the water will simply stop circulating. If a hose broke there would be a flood but the same holds for sump plumbing.