Recirculating issues

Ok guys looks like I'm in need of some more of your great experience. I have worked pretty hard to find parts and put together what I thought was an awesome little DIY RDWC, was running the system for a bit to ensure everything is solid before putting my girls inside. However as I had feared may happen, my pump sucks from the system faster then gravity can push it back from the Rez.
A few side notes before I get to my actual question. All of the line in the system is 3/4 flexible pvc, I was not able to get uniseals anywhere in my country and the only sources that I found who'd ship them wanted a ridiculous price. My pump is a 264gph Eco pump, when running it goes for about 2 min then starts sucking air, if I shut it off it only takes 20 seconds for the buckets to equalize again. Originally this was a 4 bucket design with a 20gal external Rez, but because I have six strong healthy plants in a little bubbler and haven't started flower yet I decided to put the 4 largest ones in the big net pots and the smaller one are going to stay in the net pots from the bubbler and just get moved to the 5th bucket that the pump is sucking from until I can determine sex.
So my question is; since moving to bigger lines between buckets/Rez isn't really an option what else could I do to make this system work properly? Should I drop to s smaller diameter line from bucket 5 -> pump? Should I return my pump and go for a 132gal one instead? Any ideas are much appreciated!!

A smaller pump is your only option.

Your plumbing is kinda odd. Most people return water to the control bucket,but you are drawing from it. Are your using a chiller?

I'll add, pumps are designed to push. Did I read you had it pulling water? Push the water to your plants or push it to your rez. I didn't study your design. You might try raising your plant buckets or rez bucket to aid in a more rapid return dependent on your design. Use of gravity.

That small tubing is going to make it a pain. Looks like you have an undercurrent type setup? I'm guessing the pipping going out of your room is to a chiller? Undercurrents work on gravity. Everything flows much better with larger pipping connecting your buckets and smaller tubing running to your pump. Is there a larger rez outside also or is your rez the bucket the pump is pulling from?

But yeah, smaller pump should help

Instead of having the water flow back to your buckets like an undercurrent, you can connect the first two bucks together and the line coming from your chiller have it split up high and split into each or just the first two buckets giving it a waterfall effect.

It looks like you took from an ebb and flow design with remote res and gravity feed and rdwc design and missed some elements that allow for each to operate successfully. What should you do? One way, re plumb the external rez so it is exactly like your unused bucket 5. You'll have buckets 1 and 2 (closest to the pump) connecting directly to the external rez, you'll have 3 and 4 making a continuous loop. You'll pull from the rez with the pump. You can plumb the output of the pump into each of the buckets. Or you can setup the pump to pull from buckets 3 and 4 (take that t fitting and put it in place of bucket 5), again create a loop with the external res by connecting 1 and 2 directly to it, and keep the output pushing to the res.