+1 for above comments. You need both a tank volume turnover and interna tank flow. The original pump would give you about 550gph which is probably close to the what the overflow is rated at. I'm guessing the overflow is rated at 600gph, but we need more info, like make and model or tube size.
In the Little Giant Quantum series
MDQX-SC is rated for 675 at 12'HH
5-MD is rated for 747 at 12'HH'
Hy-Drive Pond Master Pro Proline
Pro 2100-XP Hy-Drive(gph
) will give 850gph at 15' HH
Pro 4800-XP Hy-Drive(gph
) will give 800gph at 15' HH
A Blue Line 100 would probably also do the job.
You should do some online research to determine the overflow rating and match a pump to that. It may take a phone call to get the pump rating at 16' HH
The new Koralia power heads are smaller than the previous version, conusme less electricity and move more water. A couple of those on opposite side of the tank pointing upwards and towards each other should give you plenty of flow.
I also agree that a YT is the only tang you should put in that length tank.