Caution! Just because it says indoor/outdoor use does not mean it is pond/aquarium safe. Some of that stuff can leach dangerous toxins into the water. Read the label very carefully and look for the words pond/aquarium safe. If not there I would contact the manufacturer and verify.
If you're trying to seal that pipe into that black box in the picture, do it right and get a bulkhead fitting. No goop needed. Bulkhead fittings are inexpensive and will give you a positive seal. One will cost you maybe $10-$15 depending on the size.
The money you spend on various glues, silicone, etc., would pay for a bulkhead fitting. The difference is, the bulkhead is pretty much guaranteed to seal and stay sealed, the goop is not.
They are made in two varieties. Slip, where you glue your pipe into it, or threaded, where you screw in your pipe into it.