Fish voicing:

No two Fish pre-amp are alike.  As Reinhold Bogner was manufacturing his pre-amps out of his garage over the span of a few years, the designs kept evolving and components changed with each re-design.

There are two modifications that we currently know of:

The "Jerry Cantrell" mod:

Jerry Cantrell is probably the most famous user of the Bogner Fish.  Jerry used the Fish on many Alice In Chains records but mostly for live work.  Jerry had at least two Fish pre-amp modified by Reinhold.

The "Jerry Cantrell" mod is a "mid boost" mod and consists in replacing the "Shark" channel's "Bright/Dark" toggle switch by a rotating knob allowing Jerry to control the amount of mids on the "Shark" channel.  Somewhat like keeping a wah-wah pedal open.

The "Anthrax" mod:

Not much is known about this mod beside the fact it seems to add a lot more gain to the "Shark" channel.