Ditto.having to refill a 5lb bottle in 3 weeks sounds like you had a big leak somewhere.
I run my co2 into the intake of my Eheim 2128. 100% diffusion. The first time I set it up, I had a small leak, had to refill in about 4 weeks, now I get several months.
However, now that I mention it, it's roundabout time to switch it out, and I'm out of town...here's hoping it hasn't run out.