The tanks on Nautilus are ABS, which are the easiest to repair, at least for cracks. We had a crack in the black tank when we first got her. It takes some time, but you just paint on thin layers of ABS cement, which is just ABS powder suspended in a solvent. (It has to be the cement made for ABS only, the black kind. Not the multipurpose cement). You let each layer dry...it drys quick!...and keep painting it on. It will build up as thick as you want it, and we haven't had a leak since. If its a wide crack, you can even make a paste with ABS shavings and the cement, and spackle it in there, then once its dry, layer the cement on as described above. Even a hole is pretty easy to repair, but you need a small peice of ABS material to glue over the whole with the cement. Let it dry, and layer on the cement. Easy easy!