Hmm.. LEDs will add 'some' heat to the water but, they are far cooler than other lighting options. If you rule out the canister (we'll get to that) and still suspect the lights, try some floating plants or you may be able to add more distance between the light and the water (depends on setup).
Since you mostly suspect the canister (have never used a canister personally) you could take it apart for a deep cleaning. If there is gunk or anything blocking the impeller from turning or significantly slowing it down it could cause the motor turning it to overheat from pushing the energy it's suppose to without it exchanging at the rate it's meant to. Your water moving through the canister works to keep the motor cool and would naturally absorb some of this extra heat as it moves through.
Just spitballing really, like I said never had a canister filter but they're still just a small electric motor and impeller so I'm sure they're prone to common issues any motor/impeller might get.
*Edit: in the below article, scroll down to the 'servicing your canister filter' section and it actually mentions this exact thing I was discussing
How Do Canister Filters Work? Quite often, people ask, “How does a canister filter work?” Understanding how a canister filter works may lead to a better maintenance schedule and a better understanding of what is exactly going on in that “cylinder-like plastic thing with a motor” that sits...
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