We haven't used anything in the tank and have had no problems, but the longest we had water in the tank was 2 months in Florida. I know some of the guys that leave the boat in the water add a little bleach like Pierre mentioned. I would not be concerned about this at all.
Sum, you should work for MacGregor. You are a great salesman! Is there a way to access the ballast tanks?
You say your boat was in Florida (in salt water I assume) for two months and you had no green or brown things growing in your ballast tank? How about on the hull, keel, or rudder? Any marine life there? How do you check the tank and add bleach, if needed? (My wife loves bleach.) Is there one or two tanks?
As for performance, how well does the M26s point into the wind? (That is more important to me than downwind performance.) Do you ever race the boat? If so, how does it do? Have you ever sailed with, or against, a water ballast Hunter or Catalina? How do they compare?
How does the boat do off shore in ocean swells? We get fairly large swells occasionally off the so. Calif. coast. What do you do about a motor? Do you need one for docking? How do you get your boat out of the water and up the ramp?
Is there a boat owners association, or some other place, where I can get the opinions of some other boat owners? (I prefer to talk with people who actually own and sail a boat rather than someone who's information is second-hand.)
Thank you for your comments,