Personally I'd go for the drilling / bulkheads and not have all that ugly shit coming and going through your viewing plane. At least for the drain / control elevation. It really is trivially easy to drill glass tanks.
So you are suggesting doing the hulk heads. Do you have any that you suggest getting and installing? The tubing for my pump is either 5/8 or 3/4 ID.
OK that measurement is a detail that really matters. I have found this vendor
exceptionally knowledgeable and helpful. Great customer service and they sell anything and everything you'd need. Call, don't write, and have the pump or its box handy so they can get you set up with the right size bulkheads and if necessary any other fittings, adapters, or tubing:
Pentair customer service
A couple additions or some detail to what others have already provided:
...Turn the tank so you are drilling down. ... Keep the area wet with some water. ... I tape the underside with masking tape to catch the piece I drilled out and any glass dust/water slurry that goes through the hole.
It is way easier to keep a horizontal piece of glass wet and cool, than a vertical surface. On a big heavy tank drilling on the vertical is fine, but you need to keep a little flow of water going over the grinding surface. I prefer to do this outside, on my driveway - it's messy. Whereas on a horizontal plane, you can just make a "pond". I use a ring of modeling clay and a few ounces of water.
You can either work from the outside of the tank or the inside. I prefer working from the outside for two reasons. First, the less important one - you get a little bit of tearout or roughness / sharpness along the cut edge, but only on the side opposite where you're drilling from (very similar to cutting holes in wood). I prefer this roughness be on the
inside of the tank. You can easily sand this out, and regardless it'll be covered by the bulkhead hardware, but I still prefer this to be on the inside. Second, the more important one - but I guess it's really two. It's less awkward physically, and the tank helps catch any mess. I put a junky towel in there, and atop that I set a bucket or other water-catcher. You can definitely use a piece of tape to hold the glass circle up, which helps reduce that tear-out I mentioned (as does going slow at the end), but I find it easier to just let the water pour into a bucket than to daub it (and all the powdered glass) out of my "pond". Any that fails to get itself into the bucket, is absorbed by the towel. Easy peasy.
And finally - you haven't provided any info about what it is you're trying to accomplish. Nothing. So it's hard to make any informed or intelligent suggestions about how high to drill the inflow, or even if that's what would be best. Or any other details that might be important / useful, like having a sump & putting a gated wye on your inflow tubing, to regulate the violence of flow back into the tank. I'm just assuming you have a canister filter and not just a pump, since you framed all this as a "water filtration question". And a canister filter with either 5/8 (ugh) or 3/4 tubing is going to be fairly beasty - 300gph or so most likely. Raging rapids time. Anyway - all this has been covered
ad nauseum here on DB, just use the search feature if you don't feel like a conversation.
Good luck!