The pump is a great idea! That would save me a ton of pain for my 20 gallon as well. I'm gonna look into it. One of these days I need to bite the bullet and get a 30 gallon to store DI in. I wish there was a mechanism for draining water in my basement, since the previous homeowners disconnected the washer and I've got two water lines ready to go - but nowhere to put waste water

If I did, I could hook up a 30 gallon brute downstairs, make DI, and have a steady supply for mixing.
If there was a washer in the basement and the water lines remain as you say, then the drain is most likely still there and was just capped.
I does not make sense to go to the trouble of removing the drain entirely and leave behind the hot and cold water lines.
Sewer lines do have to be capped to prevent sewer gasses from entering the home, if the drain trap becomes dry.
You should also by code have a sump pump in the basement as well that you can run the waste line to.
If your water heater or other items like a basement bathroom are down there, all of those items will have a drain of some sort.
The water heaters pressure relief valve should have a line that leads to the outside if built to code.
You can tap into that just use a brass check valve to stop the hot water from getting into the 1/4 inch drain line if it vents.
I still say just extend the tubing from where ever you currently use the RO/DI unit.
You can get 100 foot long sections of tubing and if needed use a coupling or two if you need 2 or 300 feet. Run it till its full, then coil up and store the tubing and your done.
Yea the line might be running through the house and all but its just a temporary thing. Even the smallest RO/DI units can produce 40 some gallons a day. So come home from work, run the line and let it rip, might take a few nights but sounds like your not in a rush anyway.
However as you said before its to start off as a fish only. So there is no real need to use RO/DI water in the first place.
Plus the fact you will have no livestock to worry about.
So fill it with a hose from the bathroom that's near the tank. Take the bathroom aerator off and to the hardware store so you can purchase a faucet to garden hose adapter. Or just run a hose from the spigot outside into the house to the tank for the initial fill.
Add your rock, sand, or whatever bio-media of choice, add your favorite water conditioner, salt, bacteria, and ammonia source and your off to the races, no need to mix anything outside of the tank as the tank will have no livestock as its not cycled.