Specific Gravity

[jv hermit]

I have been in this hobby for a year now and am still a bit confused about which specific gravity to keep my fish in. I have a 90G FOWLR tank. When I first bought my tank, the very experienced lfs owner told me to keep my SG at 1.016. He told me to keep it at 1.016 for several reasons: The fish would not be stressed, a higher SG would just be a waste of salt, and most importantly parasites would be kept at a minimum because they can't make it in less saline water. He keeps his fish at 1.016 and they are the most healthy, colorful and eating fish of any fish store within 50 miles of my house. And believe me I've been to them all! I've read a lot of books and they all have different opinions on the subject. The book from Bob Fenner even says that there is no perfect salinity and that commericial fish only systems are routinely kept at a lower salinity to keep parasites down. My question is this. I know that reef tanks should be kept at a higher salinity because the invertebrates need it. But for fish only systems do you believe in keeping the SG lower to combat the ever present parasites that live in our tanks? Are there any other advantages to keeping the specific gravity at 1.016? :notsure:

 

[snipe]

low SG in a fishonly tank is good it will cut down on parisites that would be the best reason to go low SG. If you want to raise it but are worried about parisites I would get a UV sterilizer it will kill parisites for you and you dotn have to worrie about the SG being to low.

 

[jv hermit]

I do have a 15 watt sterilizer and my fish still got ick so I don't think a UV is a cure all for ick. (SG was 1.021). Does anyone else keep their fish at a SG of 1.019 or lower? Please advise if you ever had a parasite outbreak. I'm trying to get some evidence that a lower salinity will prevent or reduce the chances of a parasite outbreak.

 

[snipe]

I found a site on it a while back but I cant post the link sorry.

 

[ophiura]

For a fish only, 1.019 is not bad...I am not sure about the need for 1.016. You will not have any clean up inverts like snails or hermits. It is just too low. Salinity that low kills invert parasites, and those inverts we may want to keep. But I think it is extremely important that you assess why your fish got ick.
How many fish do you have?
What kind of fish?
How old is your tank?
What are your water parameters?
Did you put your new fish in a Q system before adding to the main system?
The parasite that causes "ick" is commonly present, but healthy fish can fight it off. Stressed fish start to show problems. So you need to assess what may have caused the stress, and not just trying to keep salinity so abnormally low that the parasites don't survive. And specific gravity that low is pretty abnormally low for these fish. There are also people who have extremely "healthy" fish in extremely high numbers - the fish show no aggression and people think they are happy, when in reality they may be so cramped that they can't show much aggression.

 

[jv hermit]

Thanks for the replies. I think the ick problem could have been caused by a few different things. 1) A little agression from a Blueface Angel that I introduced about 3 weeks ago. 2) A 2 hour power outage a few days before the outbreak. 3) A possible build up of dissolved organics due to a problem with my protein skimmer pump which has since been resolved. My tank currently has 1 Blueface Angel, 1 Semilarvatus Butterfly and 1 Flame Angel. I lost another Semilarvatus due to the ick outbreak. So you wouldn't go any lower than 1.019. Are you saying that parasites cannot live at 1.019? Ammonia 0 Nitrites 0, ph 8.1-8.2, nitrates 10, alk 10 dkh. All of the fish were qt'd except the Blueface Angel. I was told he was "clean".

 

[sammiefish]

just a thought...
I wonder if the parasite CAN survive in lower SG once acclimated... this would explain the ich in your tank.
Could the parasites be killed off due to the osmotic shock of transferring the fish "abruptly" (quickly enough to kill the parasite, but not so quickly as to kill the fish) to water of 1.006 for a short time then returning the fish to normal salinity, as usually described/recommended?
Your observation of "healthy" fish at reduced SG may be a result of:
Less stress on the fish to maintain their osmotic balance (easier for the fish to keep water in), therefore freeing up "fishy" resources which makes additional energy available to the immune system, enhancing its ability to deal with these "bugs". Or even just making the "extra energy" available to skin pigment processes... or whatever you want to pick
my point being: lower the SG long enough and the bugs, diseases, will probably find a way to survive... after all, the fish did... If you keep the fish at the normal SG, then you have the ability to place it in a low SG environment and maximize the osmotic gradient to most efficiently kill parasites.... keep the fish in low SG, get a parasite, you dont have as much "room" to lower the SG, therefore lowering the effectiveness of the treatment... thereby removing it from your list of available possible cures...
IMO, keep your fish at "normal" SG...

Hmmm....

 

[sammiefish]

And BTW.... I hate when someone tells me that a fish is "clean". Especially when it is someone who wants to sell it to me!!!

 

[lion_crazz]

Research has shown that fish that live in a lower SG live a shorter life than in the wild, where the ocean's specific gravity is 1.025. I really could not say how well your fish will live in water that is 1.016 but I can tell you my personal experience of lower salinity.
When my salinity gradually dropped to 1.010, my volitan lion and banggai cardinal stopped eating and went on a feeding strike for four weeks. Granted, I did not know of the salinity problems because I was still using a hydrometer (which was reading 1.022.) However, as soon as I raised the SG back to 1.022, the lion and cardinal went back to their normal habits of eating. I had had the two fish for 4 or 5 months now and never experienced a problem until the salinity dropped. Now that it is back up though, they have not given me any problems since.
I can't say that all fish are like that, but I have read from a few different sources that it does lessen their life span in captivity. How much it lessens it probably depends on how low you are keeping the SG.