Filter Question

[thewizzrd]

At the moment I have a 75g Freshwater tank that I plan to convert to saltwater in a few months. I have a fluval 304 filter and also a whisperer powerfilter. I was hoping that it shouldn't be a problem to switch the over to saltwater when the time comes. Am I correct in assuming this or do I need to get new filters? Also, with teh addition of a skimmer, this should be more than adequate filtration for a 75 gallon tank correct? Finally, one last queston, Do you reccomend an under gravel filter or not?

 

[thewizzrd]

Adding power heads was also on my list. So all of that combined should be adequate, but teh switch from FW to SW shouldn't effect them should it.