I acquired two F15T8 aquarium hoods. The tubes do not light.
They work in other fixtures. There is voltage at the ballast, the sockets are not bent.
It must be the ballast, right?
But both of them on different fixtures?
The ballast is marked:
Cat IG13-20EL
0804Y6
Thanks!
How old are these fixtures? Do they use starters?
I would guess them to be less than four years. There is a small amount of corrosion on the wiring under the wire nuts.
There are no starters, although the fixture I tested the tubes in has a starter.
IME, the lifespan of electrical components in aquarium hoods is much shorter than usual due to the excess humidity, so I wouldn’t be surprised if the ballasts have failed.
Do fluorescent fixtures that small use capacitors? We were told recently by a lighting company that our business’1000w metal halide light poles needed new ballasts because installing new bulbs didn’t solve the issue. We investigated further and found the ($150 per unit ballasts) to be fine when we replaced the capacitors, which were like $8 apiece. $150 to rent the lift truck from Home Depot and we saved an enormous amount of money.
Not unless they are internal to the ballast.
A quick Google search shows that the IG13-20EL is an electronic ballast, so there isn’t any capacitor used (in the sense that magnetic ballasts use them); rather, there is a switch-mode power supply (which does have capacitors), the most likely reason for failure is moisture getting into the circuitry, which is more likely than with a magnetic ballast, especially the smaller ones that I have seen (which are usually covered in epoxy with the connections the only likely source of corrosion; presumably, there are electronic ballasts that are also encapsulated for use in moist environments but I haven’t seen any).
Also, you say that they were “acquired”; what exactly does that mean? Garage sale? Friend? If so, I wonder why they gave away a defective product.
A 55 gallon aquarium and accoutrements on Ebay. We drove to Texarkana from Little Rock to pick it up. $125.00 for an almost new 55 gallon is worth replacing the lights. 
I ordered ballasts for $30.00. We’ll see how it goes.
You can not change parts on smaller ballast. Also it took me a few years to learn to just change the complete ballast. There are 3 parts to most metal halide ballast. The cap, the starter, amd the transformer. My experience was that if only one part is changed you will be back to change the more parts.
The ballasts were indeed bad.
I found ballasts with the same dimensions and mounting holes sealed in plastic instead of pressed metal for $30.00.