A very simple DIY fish trap I recently made,consists of a plastic glass of water,a plastic cup for glasses of water(such as these that are used in Mc Donald's orangade or Coca Cola) and two pieces of monofilament fishing line.You drill two small holes at the upper part of the plastic glass,near the plastic cup which sits on it.Each hole must be at exactly the opposite point of a diametre of the plastic glass.You put the plastic cup on the plastic glass and you drill two holes on the cup,near its edge and above the holes of the plastic glass.In the first point you tie the cup to the glass using monofilament line.At the second point you tie the monofilament line only at its end in order not to move through the hole.Thus,you run the monofilament line through these two remaining holes and you can handle its other end out of the aquarium.By pulling this end of the line the cup closes and seals the glass,thus trapping any fish that is inside it.It is obvious that you must put a fish food inside the glass in order to lure the fish you want to catch.You could also put a small thing with big weight into the plastic glass so as to stay at the bottom of your aquarium.