My friend who works at the pet store said to get rid of the cloudiness in my water I could also get some biological media. If this is true what is the best kind or brand to get? And is it safe to apply with the fish in the tank?
your tank has to finish cycling, some things will speed this up, biological media will probably do the trick (I have never used it, so get someone elses opinion), but cycling will happen on its own, your piranhas are not all that likely to die, it is just not ideal.
To keep your water crystal clear, active carbon is the best filter media. It removes tanins from the water, which make the water tea-coloured. But carbon only provides chemical filtration: it converts certain (harmful) chemicals into less dangerous ones.
When you use carbon in your filter, you must add those ceramic tubes (or bags with bio-filtration media) as well: the benefial bacteria will live on those. And they are necessary for a healthy tank, because they are responsible for bio-filtration: those bacteria convert dangerous ammonia into nitrite (dangerous for fish as well), and other convert nitrites into nitrates (not as dangerous as the other two; nitrates are removed by doing water changes).
I use carbon (and ceramic tubes) at the moment, and they keep the water very clear and balanced. But I'm thinking of replacing the carbon with peat or blackwater extract, which add nutrients to the water and make it more tea-coloured (like many S. American rivers are).