Someone here on APUG posted a wonderful suggestion, and I've followed it ever since.

I mix up an intermediate "stock" solution using 70% Isopropyl alcohol. I keep that in a 250ml bottle. I then dilute that stock solution 1 part stock to 20 parts water just before use. I mix it up in a 1 litre container and manually add the film to that container after taking the film off of the reel.

I only reuse the working solution during a single day or, in extraordinary circumstances, overnight.