Did I mention I loved the art in DoC? Even though got squashed on the PAL version, rather than re-drawn, I think I fancied Katharine (I was young).
For collisions the rod approach is pretty much how real life works, people (and creatures) do vector maths in their heads (to a varying degree - drive round the M25 (London ring road) sometime for examples - not recommended!). When odd things happen, like a planned change of direction results in an unexpected conflict, people (and I think fish) tend to stop, look a bit surprised, and then continue.
People tend to have a zone around them they are conscious of other people entering, perhaps something like that? Maybe the fish need a speed control based on local density of fish and whether they are likely to change direction (or significant direction change likelihood should be based on local fish density and fish speed, um that's saying a Fish moving quickly will tend to turn only if in lots of open space, else will slow down), again I think that's a bit like real life. Could extend to make turn direction choice based on fish density in that direction.
On top of that could have a linked direction thing, where individuals have small individual fluctustions around their shared directions for schoals. Probably needs 3 directions - global, local and individual, with decreasing degrees of coarseness, so bits of the shoal can move around the main one to a degree. Okay, this is too complex and my brain now has an essay building up in it, plus the tank is a bit small for a very big bunch of fish, so I'll stop before I bore people rigid. I hope it makes sense, some bits could perhaps be explained better in a list.
Plus I haven't had breakfast - which means no coffee yet this morning. Could be fatal.
If any use do whatever you want with this, I have no problems with everybody ignoring it either.
It's John, BTW.
P.S. Essential language guide for people who want to claim to speak English rather than American:
http://www.jet.org/FLJETAA/CONFREP.htm