Unstable tiger does not mean it's stressed... A stressed tiger is one that is being beaten up or bullied excessively or kept in poor water conditions. Yet remain stable.
I have had many tigers that get bullied badly by the bigger ones, yet mainly stable.....
U may have to accept that what ever u do, it may always be unstable.
The more ITs you buy , the more chances you will get a stable one.
They can be an awquard fish, especially if your sole purpose of keeping tiger is for the stable appearance....