Im having a VERY similar problem except my tank is newer. I can suggest an increased number of snails (DIFFERNENT TYPES... You will eventualy find some that have a taste for Hair Algea). Other than that specific things that have helped me include:
PhosBan Reactor
RO/DI water changes
Change your lighting
Increase flow (Much harder in a Seahorse tank
)
I saw you were looking for a Canister filter for extra flow instead of that get a HOB Fuge if possible and grow Cheato this will absorbe Phos and Nitrates like CRAZY
The only other option you have is to take everything out and scrub it but that can be very stressing to a set up tank
.
Levi