i do waterchanges everytime the ocean isnt rough for me to wade thigh deep.
i do 10 gal watechanges on a 40B.
i got a wet/dry vacuum from craigslist for $10, i use the wet/dry to vacuum water from my dt, then pour the 5 gallon buckets to the DT to replace the water.
there was this week in february when the ocean was just clear and no waves for 2 weeks straight, i did waterchanges everyday of 10 gals during that stretch. i live 5mins form the ocean.
from what i hear people say that you have to go 5 or so miles out to sea to get good water. have you had any problems with the tank with sea water. also how long have you been doing that?