Small Aquarium Temperature Fluctuating for Betta

Question
I usually exhaust every answer I can find before I post a question myself.  However, all questions and answers I could find about temperature changes were from people that didn't have heaters in the first place.  

We just purchased a Marineland Eclipse Hex 5 aquarium with our new fish.  This is our third fish in the last few months, because I'm sad to say the other two were inadvertent "practice" fish.  We didn't know enough of about Betta's to give them everything they needed.

Now we think we got it down (thanks to this site, actually).  I tank is about a week old.  The Hex 5 has a built-in filter system, and we purchased a heater to go with it.  We're just beginning the cycling routine, and monitoring the ammonia levels very carefully.  Our fish acts happy, so we're not dealing with a problem with the fish itself.

The problem is that our aquarium's light seems to have so much influence on our tank that the heater seems pointless.    I have had to turn the heater way down, because a few times after the light has been on for a few hours, the temperature rises up to 84 degrees; which isn't a horrible temperature, but I'm afraid to see how much higher it would go.  When I turn the heater down, the temperature stays at a constent 82 degrees.

On the other hand, when we turn the light off for the night(because the tank is in our pre-schooler's room), the next morning's temperature reads 76 degrees.  Now, these temperature's don't seem to be way out there, but I'm concerned about the daily fluctuation.  Obviously, the heater (if I had it up high enough) would use its thermostat to manage a regular temperature, but the light/heater combo will cause it to skyrocket.  However, we have live plants that we put in the tank, so I don't want to keep the light off all the time.  Is there something I'm doing wrong here?  I haven't seen anybody else post a problem like this, so it makes me wonder if I am.

Answer
Hi Landon;

It is a common problem in small tanks. It can be difficult to figure out how to regulate the temperature sometimes. With a few "teaks" though, it can be done. The light bulb in the 5 eclipse is incandescent and puts out a lot of heat so it will indeed raise the temperature, 6 degrees in your case. That much fluctuation is very dangerous for your little friend. You might consider moving the tank to a cooler location or setting up a small fan to blow across the hood. What you could also do is raise the little feeding lid and put a piece of cloth or a fish net over it instead so your fish can't get out but the hotter air can. The heat can escape more easily if it's open. Sometimes just lifting a corner of the main hood and propping it up with a chopstick, popsicle stick, etc., will allow enough heat to escape without your fish jumping out. Just be sure the open place is narrower than his body.

Also check to be sure the light bulb is a 15 watt. That's what Marineland equips them with normally, but sometimes the wrong bulb is packed in a kit. The next size up is a 25 watt and it's just too much for that little tank. Look at the area between the threads and the glass right on the bulb. It's usually imprinted there right into the metal of the bulb base. You can also get a mini-fluorescent from your local fish store to replace it. They put off much less heat and are very good for plant growth. Look at the label of the fluorescent bulb to be sure it will be safe in an enclosed hood like that.

Let me know how it goes...

At Your Service;
Chris Robbins