Using a deep fryer for fish Does using a deep fryer for fish effect 
1: the taste of other things fried in the oil. 
2: how frequently the oil needs to be replaced or stored between uses.

In short do I need separate oil for deep frying fish and vegetables, pastries... ?
Yes, Yes, and Yes.

Citations:


If you plan on offering seafood as well as other proteins you will want a separate cooker to prevent the dreaded fishy taste transfer.
Typically, once you fry fish in the oil, it tastes like fish, which might not be a flavor you want in your fried chicken.
A fish and chips concept risks not only rancid flavors from frying oil overdue for a change, but also unwanted odors—and more importantly proteins—from fish and breading that could have cross-contact with the fries.