At the options screen type these codes: 
 
 Code - Result:
 
 CHANGE - SWITCH TANK 
 RETURN - SWITCH TANK 
 ENERGY - INFINITE CRYSTALS 
 GRAVITY - LOW GRAVITY 
 HEAVY - HIGH GRAVITY 
 NORMAL - NORMAL GRAVITY 
 SKIP1 - SKIP 1 LEVEL 
 SKIP2 - SKIP 10 LEVELS 
 EASYKEY - GET THE KEY 
 DARKNESS - NO LIGHT 
 BLEAK - HEAVY FOG 
 MANIC - GO FAST 
 REFLECT - TANK REFLECTS 
 RAPID - RAPID FIRE