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I am working on starting up my tank and getting the SG up to where I would like it, around 1.025. I have no living items in the tank right now I am using instant Ocean salt and mixing that with RODI water When following the directions on the bag I am getting around 1.020 SG. Should I assume that the hydrometer is correct and add more salt? It is a swing arm type, does it need to be calibrated in any way? Also can I just add salt straight to the tank since there is nothing living there yet? I also do not have a heater in the tank yet, does this affect SG and if so, how much?

ps. I know I really should stop being cheap and buy a good refractometer but since I'm just starting, pretty close is pretty good and cheap always wins.
 

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I am working on starting up my tank and getting the SG up to where I would like it, around 1.025. I have no living items in the tank right now I am using instant Ocean salt and mixing that with RODI water When following the directions on the bag I am getting around 1.020 SG. Should I assume that the hydrometer is correct and add more salt? It is a swing arm type, does it need to be calibrated in any way? Also can I just add salt straight to the tank since there is nothing living there yet? I also do not have a heater in the tank yet, does this affect SG and if so, how much?

ps. I know I really should stop being cheap and buy a good refractometer but since I'm just starting, pretty close is pretty good and cheap always wins.
hydrometers are not accurate. i used one in the beginning and found it was off when tested against a refractometer. i would mix your water to the middle of what the hydrometer considers " the good zone". that way even if its a little low, or high, its ok. imo hydrometers can be used for fish only tanks, but when you are doing a reef you really want your salinity right near 1.025 and can only trust testing that with a refractometer. you can get them cheap on ebay. yes you can add salt directly to your tank if there is no living things in it, and from what i have found the warmer the water the easier and quicker the salt mixes. just add some salt, and let it circulate for several mins before testing.
 

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I used to have an Aquarium Systems swing arm that always read about .002 low compared to a refractometer IF I faithfully rinsed it in RO/DI water and stored it upside down. If you clean it always and compare it it will probably get you buy but you really want to get a refractometer before you invest a lot in livestock. That and a pH meter are really needed to maintain water quality. pH drop tests and dip strips are not accurate enoug IMO for a reef tank.
 

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Also if you've got any doubts take a sample to your LFS they should have a refractometer and be more than happy to check it for you.
 

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If i follow the directions exactly on my slat mix, it is always low. I use coralife salt and 1 cup per gallon only gets it up to 1.021. I use 5 cups for 4 gallons and that gets it around 1.023-1.024 so you just have to play with it. I also would mix it for several hours before testing it.
 

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One cup per gallon is getting you to 1.021??. I have never tried coralife but that seems like alot of salt.

Also when people say 'hydrometers are not accurate' they are either generalizing for new people to not overload them with information or just repeating what they were told and its just as wrong the second time around.

Refractometers are a better piece of equipment because they are easy to use, however if you spend 12 dollars on a good floating hydrometer it will be just as accurate. Just much harder to use because you need to do something like cut the top off a supplement bottle and then fill it with water, let it still, then put the hydrometer in it.
 
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