Not sure if you are considering this, but keep in mind the true amount of water you have in the tank. When I did this for my 180g tank, I figure out that I really only have like 150g or less in it. Ultimately I considered the following:
- Internal dimensions of the tank
- Where my water line is running. If your tank is 30in tall, internal dimensions is probably 28in and then you are probably running your water like 1/2in to 1in below the top regardless.
- How much decorations you have. If you have lots of substrate, rocks, etc you are never going to hit the full amount
That may give you more piece of mind. If you have room/space you can always put an emergency drain at the top of the sump that goes to another container or drains somewhere. If you do this you just need to make sure you always have enough water to prevent your pumps from running dry.
- Internal dimensions of the tank
- Where my water line is running. If your tank is 30in tall, internal dimensions is probably 28in and then you are probably running your water like 1/2in to 1in below the top regardless.
- How much decorations you have. If you have lots of substrate, rocks, etc you are never going to hit the full amount
That may give you more piece of mind. If you have room/space you can always put an emergency drain at the top of the sump that goes to another container or drains somewhere. If you do this you just need to make sure you always have enough water to prevent your pumps from running dry.