i must admit i have never heard of Rainbows being, over territorial,
They're better behaved than RTBS, but the entire genus has the same behavior patterns at some level or another. RTBS have the worse reputation of the two.
but you miss my point, both fish lived together for 12 months, with not one problem. the moment i changed the tank layout, problems occured. as such, any change or alteration may cause your shark to act differently.
What may have happened is as they grew up, you "lucked out" and they settled into stable territories. When you rearranged the tank, you broke up their territories, and they tried to establish new ones, and now they can't agree on borders. I did the same thing with a pair of julies and a brichardi - moved some rocks around, and the brichardi went nuts. I've heard of using the same means to erase territories to stop aggression, but when everything's going good, I guess it can have the opposite effect.
my RTS plays chace with the bala shark, but is never agressive, with anyone. given a 50ukg tank and not too heavy stocking, RTS even lives happyly with a Green Terror
They should do pretty good with the likes of green terrors, I'd think. Chinese algae eaters do well with aggressive cichlids, and they're also generally maligned for the ticking time bombs they tend to be in a community tank. Cichlids like GTs can take a beating and will give one back, so a problem fish will learn its place or be taught its place. Chasing is also harmless, which is why I asked about tankmates - if it's just chasing, it might not be a big deal, and if it's occasional scuffles with a robust tough-guy fish, I also wouldn't worry unless it was causing stress or injury. If there's slow or delicate fish like angels or gouramis around, that's a different matter, and once a fish turns bully with that kind of tankmate, I usually don't trust them to stay together anymore.
Not sure how it would work with a shark, but just for the sake of relevant discussion, I've found that having a fish around big enough to pose a credible threat can keep some problem fish in line, too. I have a killifish who used to be the dominant fish in the tank, and would randomly start hounding a single fish for hours for no apparent reason, once continuing for several days until the victim died. When I introduced my male keyhole, he became very peaceful. The keyhole doesn't bother him (I've seen it back down from a fight with a snail - for cichlids, keyholes are pansies) , but the killi won't assert himself when there's a bigger fish around. When all the bigger fish are out of sight, it's open season on danios, but it doesn't last as long as it used to before a gourami or cichlid wanders into view.
