Advice? Can I run extension cords on my black boxes?

I run my entire system on a extra heavy duty extension cord to a dedicated outlet. (Don't tar and feather me). My entire 75g system uses about 14 amps on a 20 amp breaker. I think you're fine if it isn't flimsy or too long, just get something rated for what you need.

 

For that specific setup I don't have special knowledge of. Just in general I think if its a few feet and heavy duty enough you're fine. LEDs aren't a giant load. What kind of pumps do you have?

 

Extension cords are rated by load based on the gauge of wire. Don't use a thin lamp cord to run your high output lights. So, that being said:
Add up your amp loads and use that to figure gauge of cord. Second. Check your cords a few times. If they feel warm your pulling too many amps through that cord. They should be room temp. What causes the heat is resistance. The more pull through a given gauge wire the more resistance = more heat = fire hazard. I can take a picture of a 16 gauge cord the wife used to run a oil heater. Toasted the plug.

As a note, manufacturers will have notes on equipment to never use an extension chord. This is their way to releive liability from someone using an extension chord rated for 4 amps on an 8 amp oil heater and toasting their house.

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And length! Guage and length. There are charts