[JmeJReefer]

Originally Posted by asylumdown

I respectfully but completely disagree. I have a 4 gallon coral only pico tank that gets a 100% water change every week. I don't even bother matching temperature all that carefully any more, though I wouldn't recommend that to someone. That tank is thriving, and is the second tank I've maintained that way, and I got the idea from Advanced Aquarist's EcoReef One, which used the same method.

I recently saw an article on... reef builders I think, about a man in Australia with a full blown reef who's colour blind and can't read the colours of a test kits, so instead of testing for calcium and alkalinity, he just does 90% or more water changes on his large reef system. It looks like the only reason he doesn't do 100% changes is because he needs to leave enough water in the bottom for the fish. That system could have easily won any number of forum tank of the month contests.

There was another article on reef builders recently championing the benefits of very large water changes as they can single handedly fix any number of water chemistry problems.

People are always afraid of 'shocking' their systems, but I have yet to see any good evidence of a case in which a parameter difference other than temperature or salinity (and even those seem to have a pretty forgiving margin) could lead to any sort of harm. Maybe pH, but the differential in pH you'd need to seriously harm or 'shock' most things is going to be larger than what you should ever have between the old water and the new.

Seeing as the only thing in this system has already survived a trip around the world in nothing more than wet newspaper, I'd say the risk to doing a 100% water change with a good quality salt that is matched in salinity and temperature is zero. If it were me and I had a heavily loaded SPS system, I'd probably still do a 100% water change as it's a perfect opportunity to reset the chemistry, and any imbalances in chloride, sodium, and sulphate that may have developed over the course of dosing mag, calcium, and carbonate.

The risk comes from the amount of time things spend out of water, or in a container that isn't heated and has no aeration.

+1. I change 90% of my water in my picos. Have had no ill effects. No losses to date. Water changes are the life-blood of this fascinating hobby. IMO.