Quote:
Meaning of a gallon
The gallon is the amount of water needed to sufficiently dilute the waste level the fish creates, to enable it to be comfortable, until the filtration can break the wastes down into less poisonous substances. So, a 1" long molly will need a gallon of water to dilute it's urine, CO2, and fecal matter to a level it can "deal with" until the wastes get carried to the filtering process (whatever that may be). A 1" damsel will need 3-5 gallons of water for the same "comfort level".
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Just to add a bit more to this explaination:
Fish skin is semi-porous and is affected by osmosis. Both saltwater and fresh water fish have an internal salinity (if this is the improper term I appologize, I'm not a biologist) that is different from their environment.
Fresh water fish have a higher internal salinity from their environment so their bodies are constantly bombarded with excess freshwater that tries to dilute their internal chemistry. To cope with this they drink very little water and produce a lot of very dilute urine to expel the excess water in their system.
Salt water fish on the other hand have a lower internal salinity so the water in their system is constantly being lost to the external environmentwhich could cause them to dehydrate. To cope with this constant loss of water, they drink a lot of water and produce very little and very concentrated urine.