Marine Aquarium Acclimation: Bridging the Specific Gravity Gap

Compensating for different specific gravityIn today’s post, I’d like to address a very common issue marine aquarium hobbyists encounter when purchasing livestock (particularly fish) and offer a simple method for addressing it. The issue in question is how to deal with the dramatic difference in specific gravity (SG) that often exists between dealers’ display tanks and home aquariums when acclimating new specimens.

As every experienced hobbyist knows, dealers often keep the SG of their fish display tanks well below that of natural sea water—often in the vicinity of 1.020 or lower. While being kept at a lower SG is in no way harmful to the fish, it can present certain challenges if the tank in which they will ultimately reside is, say, a reef system with an SG closer to 1.025. Fish should never be subjected to such a precipitous increase in SG during a single acclimation session (Saltwater Smarts contributor Jay Hemdal recommends avoiding any increase in SG over .004), so you have to choose a way to safely bridge this gap.

There are numerous approaches you can take to achieve this objective, but the method I prefer is to take advantage of evaporation during the four-week quarantine period. Here’s how:

  • Before purchasing a specimen, contact your LFS or online retailer to ask where they maintain the SG in their tanks. Don’t leave this to chance. You don’t want to end up scrambling to alter the SG in your quarantine tank when you should be acclimating the specimen if you can help it.
  • Adjust the SG in your quarantine tank so it matches that of the dealer’s water. Start with the water level a little on the low side so you have wiggle room in case the SG of the shipping water turns out to be lower than claimed/expected, in which case, you can simply add treated fresh water to the tank to lower it.
  • After receiving the fish/bringing it home, acclimate the specimen using these 10 steps.
  • Allow evaporation to occur naturally, but don’t compensate for the water loss by topping off with fresh water. Instead, carry out frequent partial water changes (I usually perform small changes daily or every other day when I have fish in quarantine as a hedge against ammonia spikes) using replacement water with an SG matched to the level measured in the quarantine tank just prior to the change.
  • Check your SG daily. Once the desired level is reached (i.e., matching that of the display tank), you can begin to compensate for evaporation with RO/DI or otherwise purified fresh water.

Again, this is just a method you can use to bridge a significant gap in specific gravity—not the method. If you use a different approach and are happy with it, please feel free to share it with your fellow salties in the comment section below.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. great advice… I wouldn’t just go on the word of the LFS on what Salinity they run though. Even if they say it’s a certain level I’d test it yourself once you get the specimen home and also test your display with the same instrument at the same time to be absolutely sure of the difference (clean it between test of course).

    • Jeff Kurtz says

      That’s an excellent point! In fact, it brings to mind an experience I had many years ago in which a dealer told me he kept the SG of his tanks at 1.020 but, upon testing at home, it turned out to be 1.016. Quite a significant difference! (That dealer is no longer in business, by the way.)

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