I'm new to all of this, please be gentle!

I've lived happily - or so I thought - with one good fountain pen and all of a sudden bought two on eBay and then four more arrived from my mother (she thinks she has some more hiding somewhere...). I splashed some cash on some Montblanc ink (Petrol Blue, Manganese Orange and Mystery Black) and have that in six of my seven pens.

The Mystery Black works perfectly with my Waterman Carene 750 nib but comes out weaker/paler with my Parker 75 with F nib.

My question is do I need to find an ink that comes out darker with the Parker 75 F nib or is it the nib by nature of being thinner putting down less ink, so if I want it darker I need to find an M nib for the 75? Is there any reference guide here with inks ideal for different nibs/pens or is it a case of trial and error? I'm not averse to finding a broader nib for the 75, it really is my favourite pen (so far).

I'm kind of answering my own question to a certain extent, this is a fountain pen forum and in a relatively short period of time I have seven good fountain pens, so different pens for different jobs. I wouldn't mind, though, having a really nice dark ink coming out of that 75...

All help gratefully received. Thank you!