I just recently bought an 8 string. I have tried every tuning I have read or watched videos on. For some reason my f# sounds to loose and there is a lot of rattle on the guitar?all the other strings sound fine???any thoughts?
On a standard scale length guitar this is a problem - you want to look at really increasing your gauge on that low string, but no matter what, it is going to be much 'flappier' than the higher strings.
You can raise the action to compensate for this or change your playing style. I play a 7 string with an ADADGBE tuning, and I have to be really delicate with the low A or else it sharpens a lot on picking, and clatters all over the place.
That said, it can be an excellent feature sound. There are a few bands who work with a loose bottom end with great success. It can sound incredibly heavy if done well.
String gauge is much up to personal preference. In my experience, the strings guitar manufacturers put on their instruments are on the thin side. Try a thicker string. There are no eight string sets that I'm aware of, but e.g. .074 strings are available from manufacturers like D'Addario. What gauge do you use for six string guitars?