just to clarify on size of reactor:
the smaller model is rated for up to 250 gallon
The model I have for sale here is for up to 400 gallon

I ran it on my 215 gallon tank. Why did I not go with the smaller model? Because if you get a model too small (and you know how optimistic manufacturers are when it comes to rating their equipment ;) you won't have any noticeable effect on nitrate. So why waste money on the smaller model and take the chance that it won't work?
I would think the model for sale would be reasonable for tanks 100 gallons and up. The media will last easily for 2-3 years so no new costs (unlike biopellets) for years to come!