Is the diarrhea truly watery runny poop, or is it more of a very soft, squishy poop? If he's having mostly normal poops with an odd squishy one, it's possible that it's a caecotroph (a partially digested poop that the rabbit usually eats to digest the remaining nutrients). These squishy poops have a cluster formation, like a blackberry or similar, but if not eaten are easily stepped on and squished/smeared about.
How old is your bunny, how long have you had him and what are you feeding? Sudden dietary changes are more likely to cause stomach upsets than simply a different environment. So a change in pellet brand, too much of a new food (greens, veggies etc), or even too many pellets can cause excess caecal poops that the bunny won't eat. If you suspect this is the case, cutting out new foods (such as veggies etc) or reducing the pellet amount should help eliminate the excess caecotrophs that aren't being consumed. Also encourage lots of hay eating.
If it is true watery diarrhea it's much more of an issue and you should get him seen by a vet. If this is the case, it's worth syringing water to keep him hydrated and possibly remove all foods except hay to get a good balance of fiber in the gut and eliminate causes.
In regards to pooping only at night, I must admit that is a little strange however Bandit poops mostly during the day, and hardly at all at night time, so it's possible that he's just on an opposite schedule.
