[tabora]

Well, I can put tape over the crack

Plastic weld it with a soldering iron.

 

[tabora]

I assume I just use heat from the gun and try and fuse the plastic together?

Yes, you can use any soldering iron, although there are purpose-built ones like this:

80 Watt Iron Plastic Welding Kit

Amazing deals on this 80 Watt Iron Plastic Welding Kit at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

 

[tabora]

Perhaps fill with water before and while attempting.

I would not recommend this. It will chill the plastic and will also get into the cracks and prevent bonding. The plastic should be clean and dry.

 

[tabora]

Now, you don't think I should plastic weld the tank after I drain it, clean it and let it dry?

No, I said don't fill it with water...