Thanks for all the reply, I think the combination of them may have caused the increase. The basement being cooler and have much better ventilation certainly can explain a lot, plus now that I think about it, the "heated" water (in the old setup) have to travel around 30ft of plastic tubing to reach the tank on first floor, maybe it was able to cooled down during that travel. The old sump also have much more surface area for evaporation.
Also, I called Poseidon and they said these pumps are WATER COOLED (!!!) and that it is normal for it to heat up the water a few degrees. But they said it shouldn't be more then 2-3 degree, and he tell me to install a fan inside the cabinet.
Anyway, we did the following:
1. I opened the pump and cleaned the impeller, there are some nasty gunks over it.
2. We removed the top lids from the tank.
Now the temperature stays around 82 degree with the light on (it was 86 with the lights off). I could shave another degree if we leave the cabinet's doors open. But regardless, 81-82 degree is still higher then I would like it to be, but I guess it is tolerable for a FO
Oh and we did verified the temperature with 2 different thermometer.