Average growth rate puts you aro at 16 to 20" within a years time. That 55g is only gonna be good for up to a 12". In about 6 months you'd better be getting a multiple hundred gallon tank ready.
Many have reported keeping silvers at pH levels above 8 with success. Myself I would lower my pH to suit the aro, don't think there are a whole lot of 8+ pH levels in their natural environment. That being said, fooling woth pH can be very dangerous for your fish if you have no idea what your doing.