Yes, I've heard that salt can change when getting old, and that you should check the SG when you change water. But listening to all the people saying they know how much salt to use, I stopped doing it. Today I found out: test the water before pouring it into the tank!.
My bag of Instant Ocean was beginning to get empty, so it wasn't brand new. I thought I knew how much salt to use when mixing the salt with water (I do this in a bucket, before pouring it into the tank. I mix the water to get a SG twice as much as wanted, and then pour half of the water needed from the bucket, and half of the water directly from the tap). Last week I tested the water in the tanks, and found that the SG had gotten lower. Ok, that means that I use too little salt, I thought. Today, I opened a new bag of Instant Ocean, and luckily I found that I should test the water before using it (to check what the SG really was when I use the amount of salt I have been using lately). Before all the salt had mixed totally with the water, I put my hydrometer into the bucket. 1.015 (this is to be used in a tank with SG 1.005)! And there was still salt left. Next test gave 1.017, and still there was a litte salt left. So the age of the salt does matter! Remember I have used the same amount of salt that lowered the SG when I used salt from the old bag. I am very glad I did test, or I would have a very uncycled puffer tank right now, and probably a puffer in shock as well!
So to all of you: test the water before pouring it into the tank, especially when you open a new bag of Instant Ocean!
