The Eheim 1000 is 1000 lph, not gph.
I have an Eheim 1000 on my 50-gallon tank, and I have a ball valve cranking it back a bit because it is too much flow through the sump. I think that return pump is quite a bit too large for a 15-gallon tank. You need to look for something closer to 100 gph at whatever head height you have imo. Be careful with small pumps like that where they might be rated for very low head height.
1/2" is pretty small for drains even if you have low flow, I think you would have a much easier time if you used 3/4" drains instead.
Also, why do you have two ball valves? Are you trying to do a Herbie drain? If so, you should only have one valve on the main drain. The emergency drain should not have a valve. The main drain should have a
gate valve, not a ball valve. You need to be able to make very small adjustments and that is very difficult to achieve with a ball valve.