Fell victim to "all knowing" Lfs

[hotsaucegsu]

Hey, I'm new to this forum and could really use some help.
Here is what I have:

- 15 gal. tank cycled with Bio-Spira a little over a month ago.
- 2 Crayfish witch I feed twice a day with frozen blood worms
- I do weekly water changes at 33% (5 gal.)
- natural gravel bottom

I went to the only fish store in town and came across a F8 Puffer and had to have him. I talked to the helper at the store to make sure my tank setup would be alright for him, and she told me that all that I needed to do was purchase a box of aquarium salt, and follow the directions on the box. This was three days ago. So, since I've come across this forum, I've started become a little nervous about his health.

Luckily, my puffer has been doing and eating fine. Tomorrow morning I am going to go buy marine salt and a hydrometer, but I am uncertain as to how I should add the marine salt to the tank. eg. How much salt? Will the aquarium salt affect this? How much water should I change?

I appreciate any help y'all can give me. From now on I will do my research instead of listening to the people from the fish store.

[Pufferpunk]

Are the crays still in there? Because if they are---they are puffer food. I'd find somewhere else to keep them. You'll need marine salt & a hydrometer to measure it with. Aim for raising the specific gravity .001-.002/weekly water change, up to 1.005 for a F8.

[Boxermom]

Welcome. I agree, get the crayfish out. Either the puffer will chew on them (even if he's much smaller) or you risk them catching him with their claws. Claws and fins never mix in my tanks, especially not with slow, hovering fish like puffers.

[hotsaucegsu]

Is there a general guideline as to the amount of salt per gallon and its effect on the SG. Sorry for all the questions this is my first time dealing with salt, and I appreciate the quick responses, the have been very helpful.
Just so I make sure that I got this right,

I'll do a 50% water change, Adding the marine salt into a bucket, and letting it dissolve, and get a reading of 1.004 before adding the new water into the tank.

[Pufferpunk]

Roughly, a cup of salt/5g will make a SG of around 1.005.

[Boxermom]

I'll do a 50% water change, Adding the marine salt into a bucket, and letting it dissolve, and get a reading of 1.004 before adding the new water into the tank.

Mixing the saltwater works best in a bucket if you have an air stone or powerhead or something in it to help move the water around or the salt will often just settle to the bottom.