5g of reactor is ridiculously large. That's what I was running and that's why I don't run it.
Don't buy into the "bigger must be better" mentality because it's NOT.
A small reactor can service a heavy calcium load tank just as well as a large reactor and it can do it more economically. The only drawback is that a smaller reactor needs to be filled more often. But a larger reactor costs more to fill, consumes more CO2 and thus drains your bank account faster.
In a downflow reactor the media will only last so long before it starts to break down and causes compaction. The DJ88 design is half-downflow. You'll find the upflow compartment doesn't get compaction and will get used up faster than the downflow (due to the partial fluidization of the media). Althought with the new larger media available this might not be as much a concern. But with the old gravel style media it was only good for about a year anyhow, whether it got completely used up or not.