CichlidFish.com - Fish That Think.

My 90 gallon tank has 2 11" oscars in it right now. I've recently gotten involved in this whole salt water thing, so have been considering the possibility of converting the 90 to salt water. I have a buyer for my two oscars from a lfs. My question is that since this tank has been running for several months and is cycled now could i convert the water to salt water w/o emptying the tank. I would take out the gravel and add crushed coral. Is this a bad idea and/or will it help the new fish that i have in another smaller tank as far as becoming adapted to the new water. Will the tank have to re-cycle after i adjust all water parameters? Thanks for your time.

I am moving this to the saltwater board cause there is a lot of salt questions.

Will the tank have to re-cycle after i adjust all water parameters?

Yes slightly,because by removing/changing all the gravel your removing alot of the established bacterial colony and some of the bacteria on the glass and etc.. will just die off from the change to salt.

w/o emptying the tank.

If you leave some water in it might help some But DON'T mix a full tank with salt (I learned that the hard way) tank out some water, mix up some salt water in a bucket or something and add it.

take out the gravel and add crushed coral. Is this a bad idea and/or will it help the new fish

If you have regular gravel now,changeing to Crushed Coral would help out a salt water tank alot since it helps keep the PH up,which you want for saltwater.

You can also use an aragonite based sand also. If you are where you can get Southdown Play Sand from Home Depot it works very well and is alot cheaper. Your tank will go through a cycle though so make sure not to add any of the fish besides maybe a couple damsels to help things out. You can put in your Live Rock though as the cycle wont' hurt it.

I wouldn't mix the water in your current tank with salt to try to convert to marine. Freshwater fish are a lot dirtier, especially oscars. There is too much waste and the water isn't clean.
Saltwater fish need near perfect water conditions to survive. A lot of people use water purifacation units like RO or DI just to make sure the water is as pure as possible, as there are many things in tap water that will kill saltwater fish, where as lots of freshwater fish could survive small amounts of them.