Ok, updates:
One thing my mom taught me was to "use what you've got." I already have an external overflow that I paid for and haven't really gotten my full use out of it yet. If I just get a mag 3 to run it, I will feel better about it continuing to run, instead of using the Rio 1100 return pump that I have now. Since I'm not planning on keeping the 20g for more than a year, I won't drill it. The next tank that I have will definitely be drilled.
So, new updated equipment:
mag 3 return pump
Octopus NWB 150 (so that I can also use it on a future tank)
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Phosban reactor
calcium reactor
ATO Unit
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glass baffles
silicone
flat white spray paint
pvc parts
Rainbow locline plumbing parts