There are reasons why a marine aquarist might want to drill the glass of his/her aquarium and often the first question is as above.
The first problem is the imagination. There is the aquarium fully constructed. The aquarist is going to use a sump and unfortunately the overflow hole in the glass has not been done before delivery, or the aquarium has been running for a good period and the aquarist wishes to upgrade the system. The imagination shows the aquarium with a horrible crack appearing where the hole should be, or the hole is completed and then looks like the centre of a spider’s web as cracks appear all round it.
When the system is new it is best to have any drilling done before the aquarium is delivered. There are two reasons – first, the person from whom the aquarium has been purchased is probably used to drilling glass, and second the guarantee will remain intact. If the aquarium is drilled by the aquarist after delivery and there is a problem it is likely the retailer will tend to blame the DIY work, justifiably or not.
If a hole needs drilling and the aquarist is to do it, then control the imagination, it is not that difficult. Using the correct tools and employing some basic simple techniques should see a successful job. Of course, anyone who is just too nervous can always ask the local retailer to pop round. Just make sure there is good access to the hole area and the seawater level can be dropped.
Below is a website that advises everything that the DIY aquarist needs to know before commencing.