Hello; I agree with the other replies, level the stand and tank as much as possible. I consider out of level side to side or front to back to not be good, but out corner to corner is worse.
Without some more specific information about the stand it is ,to me at least, difficult to comment on how shims might affect the stand. keep in mind the uneven floor is putting uneven stress on the stand as it sits.
A basic assumption on stands is that they should be pretty close to square when built.
Get a level and check the floor in the area where the tank will sit. you had a 150 in that spot for two years. That can anywhere from 1500 to 1800 pounds static load. The floor may have been affected and the extra weight of the larger tank may have made it show up that much more.
good luck