boric acid comes in a powder. you can pick it up at home depot. you can lay a line as a perimeter. the roaches dont die on contact, but they will take it back to their nests and share it with the other roaches. i use it at my house. its good stuff. just dont puff the dust too close to your tank and you are set
not sure about the safety of boric acid around tanks.. but it works as a microscopic abrasive... causing millions of teeny tiny cuts and abrasions on the bugs it touches... thus dehydrating them and killing them.