Back and forth 6 times

This is utterly shocking. I purchased a water heater for my fish tank last week on Wednesday. I brought it home and plugged it in and it worked for five minutes and then died. Took it back, was given another, same thing happened. Then went back again and was given another and asked that it be tested. Worked in the shop as soon as I brought it home same issue. Each time I took it back and asked the manager to test it in the shop and each time it didn't work there either. Have been back 4 times now, asked them to test the last one again, get home and the wire on the heater looks like it's been chewed by rats. I have now officially wasted 50km's up and down because of your faulty stuff. I am so angry I don't even know what to do anymore, I need the heater but all of your stock is completely **** even when I switched brand, wires were chewed. I won't be buying from you in future EVER.