what is the relationship between cold blood and "activeness" in animals?

Question

If crocodiles, and lizards need to warm up in the sun before getting active due to their cold blood, why then fish don`t?

Answer 1

Fishes and reptiles can not control their body temperature, and so their body temperature fluctuates with the environment. They are called poikilothermic animals. They become sluggish when the temperature drops. If you have kept some tropical fish, you would have noticed this. The temperature of the water environment does not vary as greatly as the air temperature in a day, and so the behavior of fishes is harder to perceive.

Cold water fishes have adapted to cold environment and are quite comfortable at low temperatures.

Answer 2

"Cold-blooded" is a horribly outdated term. Animals once classified in this way actually regulate their body temps in a number of ways besides the external influences of air, sun, water, etc.

Fish do indeed move about their surroundings to find a suitable temp, but for them, this means changing depth. Large bodies of water get colder with depth (to a certain point, at which you have a rapid change in temp called a thermocline).

Any organism's metabolism is governed by a complex set of criteria, including age, gender, post-prandial time (how long ago they ate), seasons, time of day, etc. It's just that some ectotherms (like the lizards) have found an "easy" way to get energy. (If you think about it, lazing about in the sun can be fairly dangerous, especially for small prey like lizards.)