It's great that you're keeping the water clean. Some fish do change color when they sleep at night, although it could be temperature fluctuations as the house changes temperature when the sun goes down that could be stressing him out a bit.
Getting a heater would be an excellent idea. You might want to invest in a tank that is at least two gallons--there are many cheap ones out there and if you're really strapped for cash, you can temporarily keep him in a sterilite/rubbermaid plastic storage bin. These storage bins are easy to clean, safe to use with any heater, and they're completely safe for fish. The best part is that a 4 gallon bin is only about $3, so you can use it for a few days or a few months and then repurpose it somewhere else in your house, or even throw it away. I use mine for hospital/quarantine containers and for dipping/sanitizing live plants. They are very useful to have around.
When picking a heater, try to find a 25 watt heater with an adjustable temperature dial. Non-adjustable pads and preset heaters are cheaply made and won't keep the temperature stable. They may overheat or underheat the water based on the ambient temperature of the room--which kind of makes them rather worthless, in my opinion. Adjustable heaters are a few dollars more, but they will last longer and give you more security, stability, and control. I use and recommend this heater: http://www.fosterandsmithaquatics.co...8&pcatid=11368
Your fish may also have been bloated from what you've been feeding him. Often that can cause inactivity and can make the fish float around a lot. I recommend pre-soaking all of the dry food you're giving him in a bit of tank water until it is completely hydrated. Freeze-dried foods especially can cause terrible constipation/bloating issues--so use those sparingly and always let them hydrate completely and break them into smaller pieces to help avoid digestive stress.
The reason why this causes problems is because fish weren't really made to eat dry, bready, air-filled processed foods. What happens to them when they eat dry food is comparable to what happens to us when we eat uncooked rice or noodles--the food expands in the gut as it takes on moisture and causes bloating and constipation. Bloating in fish puts pressure on the swim bladder--the organ responsible for controlling the fish's depth in the water, causing it malfunction. The resulting swim bladder disorder can make the fish unable to sink or unable to rise until the pressure is removed.