Hole in bottom of tank?

I would say that hole was probally for maybe a filter hose or something if it looks cleanly cut, If it looks messy and sharp then opps someone had an accident.

Ofcourse there is also the possiblity the tank didnt have water in it but a snake or rodent or soemthing so maybe the hole served some other purpose.

Easist wat to plug it would probally be a sheet of tough glss maybe 5inx5in and stick that over the hole using a really powerfull glue and silcon sealent.
You wont see it wne you have you susbstrate down so just make sure you get a really tight seal. My tank has a replaced pane at one end which is completly water tight so it is possible to repair them (just taks a little patience and a whole lot of luck

If you put a 5x5 sheet of glass over the hole, you'll probably want to make sure you put a bead of sealant around the edges of the plate after it's in place. I'm sure you wouldn't want to leave any surprises for curious fish rooting through the substrate.

I would think it would be in a corner and it could use an over flow but in the center of the tank is odd. Just don’t skimp on the sealant and give it time to set. I think it’s about 48 hours. Since it is the bottom of the tank I might go with something a little bigger than 5x5. I might try something larger to help spread out the weight of the gravel and the water since its 40 gallons. The center of a pain of glass will be the weakest with the hole in it.

When you buy glass, don’t you pay for the whole sheet not the chunk you’re wanting?

you couold get a threaded bulk head fitting and them plug the female treaded fitting in the bulkhead with a male threaded pipe plug

another thought....

You wont see the repair so why not use something like steel or even a fibre glass of some sort (it will be covered with substrate anyway.

Also I'd agree go for a large peice and leave loads of overhang to help spread the weight

You wont see the repair so why not use something like steel or even a fibre glass of some sort (it will be covered with substrate anyway.

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While this is sound thinking from a strength point, silicone will provide a much better bond in a glass to glass situation, I'd go with a sheet of glass (I'd honestly go with the largest sheet you can fit easily in the bottom of the tank. silicone the whole surface and stick it in place. make sure there are no large air pockets in the fit, and put a wieght of some kind (not too heavy of course)on it to squish it down good. Give it a minimum of 48 hours, and then clean up any excess silicone around the edges with a razor blade or just leave it since it will be hidden. Contrary to popular belief, clear silicone is "the" adhesive used to bond glass in aquarium building. there is no glue used. Acrylics are a different story. you will find that if you ever try to remove the patch, silicone is unbelievably strong.