If your tank now is 36" wide already and I'm assuming the length is higher than 72", the silver should be able to live in there for life! But, bigger is always better.
But yeah everyone already answered your question about their growth rates....I have 10 of them in a 55 gallon and got them at 2.5"-3" now only a month later they are all 4.5"-5". So they grow quickly...
