Just wanted to support the idea about the fruit in the tank. Usually not a good idea because of the sugar and fruit is less likely to stay together very long in water.
If you have a clown pleco and didnt pay much for it, kudos to you because they are about 30 bucks by me for a super small juvy. Still, they get to about 4 or 5 inches and grow similarly to the bristlenose pleco. It wouldnt be impossible, but I would think in 6 months that it would be larger than one inch. Good news is though that they can fit into a 10 gallon, but keep an eye on your nitrates since they do make a lot of waste and you might have to compensate by bigger or more frequent water changes.
Your tank does look really nice. I got the impression that your 'minnow' was a tetra at first glance too, but I am really leaning towards either a fully grown feeder guppy (had one of my own once) or a female (which maybe what a feeder guppy really is, not sure). The guppy I had was meant for an eel, but it made it somehow and looked very much the same as the one in your pic, but it was pretty much a greyish/clear color which is hard to tell on my computer screen from your pictures.
If it is surviving well and you are not adding other fish on a regular basis, then I would just keep it in the tank at a temp of 75 to 78 or so. The reason I say that is because keeping a cold water minnow in a heated tank, or a tropical fish in a coldwater tank does stress them out and they can pick up Ich easier but if you dont have Ich now and are not bringing in new fish that could be carrying it, you would be fine in either case. The pleco and the guppies need tropical waters though.
I wouldnt use a heater for a 55 gallon tank for a long time, but you could probrably get away with it for a little while. Two things to keep in mind are to make sure that the heater is fully submersed (at least the glass part) or it can burn out or otherwise not know when to turn off or on and keep a thermometer in the tank because even though the heater is set at 75 (if it has a temp guage), it probrably would keep things a little warmer in a smaller body of water. Otherwise, it should work well and still turn off and on right. It would just be able to heat water faster than it was designed for. I have a 30 gallon heater in a 3 gallon tank right now because I was in a pinch and needed to keep a q-tank/hospital tank warm quickly. It is fine but I am getting a new heater tomorrow. Walmart (if you are in the states) has 10 gallon heaters for 8 bucks near my house.
