The Puffer Forum

Hi people!

It's a question concerning Neale Monk's Brack Calc software.

I’m trying to find out the right salinity to my low-end brackish planted tank. I have 2 F8s and the experts say to keep them in a SG of 1.005 (7,4 g/L of salt). To keep plants and F8s together I should keep the SG between 1.003-1.004 (4,8-6,1 g/L of salt).

I have a Sera Hydrometer and it is calibrated to 25°C (77ºF). My tank’s temperature is 26ºC (78,8ºF),do I have to keep the SG between 1.002-1.003 (according to the Brack Calc it means 5,2-6,5 g/L of salt) to keep the plants alive and the F8s healthy?

Even though my hydrometer is calibrated to that temperature, can I take the results of my measure (in the tank) and convert it in the Brack Calc to get the REAL salinity? I know density fluctuates by temperature, for exemple: if you have SG 1.005 at 25ºC (77ºF) it will have a very different salinity if you have the same SG at 28ºC (82,4ºF).

I’m really confused how to use a hydrometer calibrated to a certain temperature.

Thanks in advance.

Figure-8s don't need much salt, and will be perfectly happy at fairly low salinities. There is no great advantage to a higher salinity, unless you have very soft water in the aquarium. In that case, extra marine salt mix will provide more carbonate hardness, and that will offset background acidification.

To get good plant growth, the specific gravity should be SG 1.003 or less; while there are a few plants that tolerate higher salinities, there aren't many. So if you're going for the "tangled swamp" look, I'd stay at SG 1.003.

Brack Calc doesn't directly convert specific gravity at one temperature to a specific gravity at another temperature. Brack Calc was designed specifically to allow you to work out how much salt (in mass) to add a litre/gallon of water at a certain temperature to get the specific gravity or salinity you want.

So, in your case, if the aquarium is at 26 C, drag the bottom (temperature) slider to 26 C. Since you want a specific gravity of 1.003, drag the top (specific gravity) slider to 1.003. The answer is given along the bottom: the salinity will be 19% that of normal seawater, and you'll be adding 6.5 grammes per litre.

Now, that's rather more salt than you need, and your plants will prefer a lower salinity. Drag the top slider to SG 1.002. You'll notice the salinity drops to 15% that of normal seawater, a perfectly acceptable level for figure-8 pufferfish. You'll also see that the salt concentration is 5.2 grammes per litre.

As you can see, while aquarists look at specific gravity because it's easier to read, it can be misleading. If you drag the temperature slider to 28 C, and then pull the specific gravity slider to 1.003, you'll see the salinity is 7.2 grammes per litre, 21% that of normal seawater.

Always concentrate on salinity, not specific gravity. Salinity is what fish and plants "feel". Since we generally recommend both marine and freshwater tropical fish be kept at 25 C, this isn't usually an issue. But if you keep your fish warmer than that, specific gravity can become misleading if you don't cross-check that value with salinity as well.

Cheers, Neale

Nice to see you're still scanning the threads, Neale!

Hi Jeni,

Only take a peek now and again, and in this case, the original poster sent me a personal message. My fish forum time is considerably curtailed these days thanks to helping out at WWM, as I'm sure you'll understand from personal experience.

Cheers, Neale

I thank you very, very, very much for claryfying the question. I'm really glad you received my PM and answer the question (that will be useful for everyone!).

My pleasure! Have fun with your puffers!

Neale

if you were trying to be very accurate in your salinity measurements, would it be better to weigh the amount of salt and water you are using and then mix it? Average salt water salinity is 35 ppt i think, so would that be 35g of salt and a kilogram of water?

Under lab conditions, yes, you can make seawater simply by adding the right masses of the constituents to distilled water.

The problem for the home aquarist is that marine salt mix absorbs water from the atmosphere so that over time each gramme of marine salt mix becomes a bit less salty and a bit more watery than you think. Taken to an extreme, you could add 35 grammes per litre, but end up with a salinity slightly below seawater.

Another issue is that marine salt mix is NOT seawater with the water evaporated off. Marine salt mix contains more carbonate hardness (alkalinity) so that over time pH changes occur more slowly than in regular seawater. So in practise, you actually need to put in slightly more marine salt mix (by mass) to get the precise salinity you need (measured using specific gravity).

The difference is trivial for brackish water fish, but for saltwater fish it can be important. Hence, while you can certainly used Brack Calc to *estimate* the amount of marine salt mix to add, you should always use a hydrometer to *check* the salinity afterwards.

Cheers, Neale