Debone and cut cod into small pieces. Place in a large bowl.

Squeeze the juice from the limes over cod. Make sure that it is mostly covered by juice.

Cover and place in the refrigerator for a half hour. Stir a couple times.

Meanwhile, dice crab, shrimp, onion, tomatoes, jalapenos, and chop cilantro

Check cod by now it should have turned opaque and be cooked by the lime juice. If not stir and chill longer.

When cod is done. Add chopped ingredients, salt, cumin, and chili powder.

Stir well and chill another 15 - 20 minutes

Plate a tostada shell. Top generously with ceviche. Place avocado slices on top. Add tapatio and crumbled cotija cheese.