Canister or HOB
...both will cause nitrate problems if not regularly maintained. The trouble is detritus building up inside the filter. If you take the floss, or filter pad out and rinse it regularly, you won't have any trouble with it. There is no denitrifying bacteria in mechanical filters, so they leave you with nitrate. Putting rock rubble in a HOB
filter will probably do the same thing as anything else in a filter- if not regularly rinsed to get all the collected detritus off.