Testing salt water with refractometers for the proper specific gravity or salinity is the easiest test for aquariums. Most Refractometers do not read accurately when calibrated at -0- with RO, RO-DI or distilled water. Calibrate the Refractometer at 35 ppt. Use a precisely formulated scientific standard calibration solution. Solutions purchased online from reputable aquarium supply company will work. In some cases a Refractometer can be off by .001-.004 points when calibrated improperly, this alone is enough to cause major problems with aquarium inhabitants.
The refractometers measures the amount of light shift, called the angle of refraction. This refraction is converted to a useful unit of measure. With a saltwater refractometer, the units are specific gravity and salinity.
For freshwater fish you may be able to get by without having one. if newly acquired fish do come down with something, you will wish that you had one ready to go. One new fish that is introduced to your main tank can easily wipe out the entire tank population. Better safe than sorry.
For saltwater aquarium keepers, I would say that you need a quarantine tank. Marine specimens are usually wild caught. Their journey to a dealers tank is stressful for them. Stressed out fish are susceptible to disease. Reef Aquariums and aquariums with live rock, cannot be exposed to the harsh medicines necessary to most ailments. Some medications can wipe out all of the invertebrates in a tank. The last thing you want, is to find out your fish in your reef aquarium are covered with parasites.
Quarantine Tank Setup
A bare glass aquarium anywhere from 10 – 50 gallons is all that is needed.
An Air Pump
Basic test kit like the API kits and a copper test kit Fish Net – don’t use the same net for your main tank
For newly acquired fish you will want to acclimate them to the water in the quarantine tank and monitor them very closely for a period of four to six weeks. Monitor the water parameters and check for signs of parasites or bacterial infections.
If the newly acquired fish does need treatment, you will want to use the appropriate medication and you will need to keep them in quarantine for the full duration required for treatment. If after the initial four weeks no problems develop, you can then acclimate them to the main tank water and then introduce them.
Save yourself some money and headaches by having a quarantine tank.