Differences
Here is what I understand about Fountain pens vs. Dip pens
The nibs are very different. Because of this, you should NEVER use dip ink in a fountain pen. The nibs look very different, also.
The dip pen nib is meant to be dipped, and retain ink for a little while while you write.
The fountain pen nib is much more complex, and is meant to be part of the feed mechanism. The pen has an internal ink reservoir (pump, bladder, catridge, whatever), a connector to feed the underpart of the nib and the nib controls the "drip" that is ocurring.
The nib has a slit and an "airhole" to allow and control that flow.
The mounting is different.
The dip nib typically sockets into a mount ... and the pen body is much more slender and tapered.
The fountain pen nib is part of a holder mechanism that screws or fits into the pen body and connects to the reservoir.