They glow whenever you can receive gems, passes, coins and charms from your aquarium - generally every 12 hours. They stop once you’ve collected the goodies.
so its when you go to collect, before you collect the prizes, the fish kind of glow, and you can click on each fish for your prizes, or hit the collect all button.
XP is your progress on when you can level up your fish. If you’re at 7/5, you can level up your fish one time. If the XP is 1/5, you would need to catch more fish to level it up.