I happen to have a lot of experience in industrial heat exchange and pumping systems, and it seems with the early GinaEric design, its 600W power supply and hand being fed, the little self contained pump and reservoir worked fine, but as more people go to higher wattage power supplies and especially adding on the auto feeders, cooling starts to become a limitation. I haven't built my system yet, so I can't give you any data to support my ideas, but I will suggest what improvements should give best bang for the buck. Also please let me be clear, I'm not at all bashing the original design. It worked great and its a decent price. but for those who want more cooling capacity for whatever reason I have some suggestions.
The coil with the 1/8" tubing gives a lot of resistance to flow. The only reference to a measured flowrate through the closed loop in the basic design that I recall seeing was by Gina. She'd measured 1/2 cup (4 ounces I presume) in a minute. That's 0.03 gpm or just under 2 gph (gallons/hr), not much flow. Lol on these little pumps I thnk they generally rate them in gph instead of gpm to make the number look bigger. Almost all these inexpensive pumps in this size and flow range are open impeller centrifugal pumps. Its because they're by far cheaper to make, and can be fairly reliable for the cost. The only flowrate related spec on the GinaEric partslist pump I saw was 1.9m and that's not flow, but head. Head is a measure of how much pressure the pump can overcome, and the units are in terms of height of a vertical column of water, and in this case 1.9m is 1.9 meters. To put it in terms more people can relate to, that's roughly 2.7 psi. With centrifugal pumps flow and head have a inversely proportional non-linear relationship. It may be closer to linear in the middle of the performance range, but gets steep on the ends. On an industrial pump you'd buy, the manufacturer would supply a flow versus head graph that had overlayed HP requirement lines. Unfortunately we don't have a curve for this pump. In my experience if you're only given a head spec for a pump, that's the pressure at which the flow drops to zero, so your system pressure drop (resistance) must be lower than that to get ANY flow at all. Electrical people can kinda think of a pump as a wierd, fixed watt power supply. If the piping system attached to the pump has a lot of resistance or ohms (friction from turbulence of going through small tubing), then it takes more volts (head pressure) to get your flow. and since you have limited watts available, your amperage (flowrate) is reduced. Ok, ok its vaguely similar.
Ok now that we're all up to speed on pump flow characteristics, I will say that I bet there is someone who's seen how much flow that little pump can do if you just point the outlet hose back into the top of the reservoir (without passing through the coil and heat exchanger. I'll wager its 3 or 4 times as much as Gina measured going though both the coil and the heat exchanger. What's happening is we're operating this little pump way to one end of its performance curve, so we don't get much flow.
The other way to increase cooling capacity of the system is to improve performance of the existing heat exchanger (radiator). Heat exchangers in general are relatively high pressure drop compared to the piping system they're installed in, and in fact if you don't have enough flow though them to the point that there IS significant pressure drop, then you're operating them on the LOW end of their performance curve. What I'm saying is that on a gpm basis, you transfer more heat from the water to the air when the water flows fast than when it goes slow. Heat exchangers need turbulent flow to conduct the heat from the body of the water stream through the wall of the tubing to the cooling medium on other side (air in our case). For a given heat load a system with an oversized heat exchanger will reject less heat than one with a properly sized heat exchanger. Its not intuitive, but this is one of those rare situations where bigger isn't better.
Its really a challenge for this one pump to give us sufficient flow through two high resistance items in series.
So now that we understand the issues, we can come up with improvements. There are several ways to approach it. If you've got the original pump with integral reservoir and aren't ready to start over, I'd suggest buying a small centrifugal pump that has both inlet and outlet connections and hose barb connections that matches your current tubing size at your radiator. There are some on ebay that are sold as solar circulating pumps for less than $10.
https://www.ebay.com/itm/DC12V-3M-2...ersible-Pool-Water-Pump-Solar-56/253130755697 You'll also need two tubing tee's and a couple of extra feet of tubing. See the attached sketch for more detail, but you basically create a recirculation loop around just the radiator that is in series with the original pump and coil. This increases the velocity of the water going through the radiator improving its overall heat transfer, and at same time reducing the head that the original pump feels. The latter shifts the operating point of the pump on its flow vs head curve towards an incrementally higher flow. The combination of the two will increase the flow through the coil and improve the heat rejected at the radiator, so if running at the same cartridge per minute rate the steady state water temperature will be lower than before.
The other option if you had built a reservoir using an external pump, you can add a second pump that will circulate just to the radiator and back. The original pump that went to the coil can be rerouted to come straight back to the reservoir, bypassing the radiator. There are some small submersible pumps sold on ebay for around $10 that are closer to 5m head (
https://www.ebay.com/itm/Newest-DC-...bmersible-Immersible-Pump-Shower/122863803456 ), so if your reservoir has enough internal space for it, you can add one of those. The system might perform better with the 5m one feeding the coil.
Hope you managed to stay awake until the end of this, and if you've got any questions feel free to ask.