The black specs may also be manganese which could change sizing and filter run lengths depending on what he has bidded for your needs. When in doubt check it out. If this is a local dealer maybe you should get a second opinion. Reputable dealers should have no problem in giving you a free quote and if the general synopsis is the same amongst all of them as far as what you are needing then you can compare pricing after that. And you should always have a water analysis done you may contact a local university and some will do the testing for free.