Here's a picture of the refill cartridge:
It looks pretty similar to standard 88g air gun cartridges like these:
The Fluval one costs ú14 (so it'll be around $20USD) whereas the air gun cartridges are only $15 for 2.
For a sub-10 gallon tank, how long do you guys think each cartridge will last?
I wonder how good the needle valve is.