Nice score. 150gal is the size i would like to wind up with. Right now im at 90...
if thats the only can filter he has (or you have) that is on that tank, id really suggest something a little more "stout" for a tank that large.
on my 90 gal i have 2 external cans and a third polishing submersible.
Even then, i needed to add a small wavemaker to keep the sediment from settling on the gravel.
i have found that there is no way around doing "more work for more water".
20-30% water change on a 55 is roughly 11 to 16 gallons, or 2 to 3 buckets.
compared to:
20 to 30% of a 150 is roughly 30 to 45 gallons, or 6 to 9 buckets.
maybe they just got tired of it or got in over their head on this one. idk.
one thing that works for me is 2 dedicated can filters. (i have no experience with sumps but that might be something to look into with a tank that large)
one canister is filter pads only. the other is bio substrate only.
this makes cleaning things easier for me. I toss the old filter pads out and clean out the canister when needed. The bio filter has a "T" connected to the canister outlet (with a few valves). to clean that one, i turn it off, close one valve and open the other, and the canister backwashes into a bucket.
hope that helps a little. Remember i only have a 90gal tank and would love to have a 150gal like that, but i can say; if i had to go from one can on the 55 to 2 cans (plus some) on the 90, then there is no way that the single can there will be enough for that many fish with that many gallons of water.