This is something I noticed from personal experience. During the summer, I took out my heaters to save on the electric bill. The tank temp dropped down to around 70-72F on my 2 tanks. My oscar and my male jd stopped eating pellets during this period, but would still go crazy over shrimp and worms. I even starved the JD for a week to try to get him to eat pellets, but it was a no go. Recently, I put the the heaters back in for the winter, so the temp is now up around 80F. Both now will eat anything I put in the tank, including the pellets. When you changed tanks, did the water temperature change? Is it as warm as it was in the little tank.