Probably a stupid question, but please hear me out.
I want to make an aquarium light controller with an arduino nano that will be turned on 24/7. I was wondering if it is even safe to leave a nano on all the time like that. Does it get hot enough to become a fire hazard? Do they draw too much current to be left on all the time? I have never used a nano before, as I do all my prototyping with a mega 2560, so all help is appreciated.
The nano in my anemometer runs 24 hours per day, 365 days per year and has for about 5 years.
The nano in my irrigation system runs 24 hours a day for the entire 9 months irrigation season.
Your nano should run without a problem.
If your circuit is designed properly, you can run Arduinos for years+
The aquarium lights would potentially be far more of a hazard than the Nano.
Oh, ok. Lights aren’t an issue. I use 12 volt LEDs, and they stay pretty cool. I assume that relays can be turned on for 8-10 hours at a time as well?
I know that elegoo has a nano 3 pack, but does Arduino have something similar? I need a few, and I feel that it would be cheaper to buy them in a pack.
Why do you need a few? For different aquaria it might make sense, for one aquarium probably not.
"In my experience, it's better to have a gun nano and not need it, that to need a gun nano and not have it" 
Just trying to prevent that OP is going to use 5 Nanos for 5 lights in the same aquarium if that is not necessary 
I’m not using them all for the same project. I have a few different projects that all use a nano, so that is why I need a few.
I figured, but I couldn't resist.
Best to buy more than you need in case of accidents. Also, you may want a real Arduino to check your clones against if you encounter funky behavior. That said, I don't recall seeing any bad stuff about Elegoo's version.
I will use a real arduino for the final version since they are the most reliable, but I will probably use the cheaper clones for prototyping.
Not a good assumption. If the contacts are carrying much DC current, they may weld themselves together. The alternative is to use a MOSFET in place of the relay.
If the relay as carrying AC, then there will be less of a problem.
Is there a certain MOSFET that I should use for this? Also, does it connect the same way a relay does?
Does it even get hot enough for you to notice when you put your finger on it?
Here are the schematics. I attached a screenshot as well as the .sch file.
Note the the ‘Light Control’ box is where the component I am inquiring about will go.
Your power LED needs a series resistor.
If that’s a 5v Nano, you cannot connect 5v to Vin.
Use circuit with Q2 or Q3 in post # 15.
I just realized that I forgot to put a resistor there.
If I don’t connect 5 volts to vin to power the nano, do I connect it to a 5v pin, or somewhere else?
I need a MOSFET for the light control, not the motor control. I have to use an L293D motor driver for that, as it needs to be reversible. The MOSFET is for the LED aquarium lights, they use 12 volts, and about 1.5 amps current draw.
The motor in that circuit is replaced by your LED strip.
Your external 5v power supply would be connected to the Nano’s 5v pin.
Do you have any MOSFETs ?
