Do I trust the hydrometer?

[5to9design]

I'm coming from the freshwater side of things so forgive my ignorance. I recently set up a brackish tank for fiddler crabs. I read on multiple sources that it takes approximately 1/4 cup or 2 tablespoons of marine salt per gallon to make the 1.010 sg a brackish tank should be. I mixed 8 tablespoons into 4 gallons of water but my Instant ocean hydrometer is only reading 1.004. Assuming I had miss-measured I dumped it out and started again. Same result. Are these hydrometers not that accurate? Water was roughly only 65 degrees when mixing, could that affect the reading that much?


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[Sk8r]

Get a refractometer. It'll save you a lot of grief. I get mine from Amazon, less than 20.00. Works fine.

 

[Vinny Kreyling]

Temp will affect salinity readings.
A swing arm can be way off if not used correctly.
A refractometer would be better, get one with automatic temperature correction (atc).
Try to mix your water closer to the temp of the tank, room temp @ a minimum.

 

[Sk8r]

Thanks, Vinny. I didn't know about the atc. I'll look for that myself---useful.

 

[Vinny Kreyling]

My pleasure to be of service Sk8r.

 

[reefteaser]

I purchased an H2Ocean refractometer 6-7 years ago with an expanded/reduced scale from 1.015 to 1.030. It's specifically for aquarium use and easier to read than the typical ones with a larger scale. I calibrate it every once in a while, but it seldom needs adjusting.
I don't know if it's still manufactured, but if you're not also making beer I'd recommend it.