75 gallons is great, and should not be too big, just make sure you have something in there for the eggs to stick to so you can take it out after (see below for more info on that).
Just to let you know it can happen, my fish chased each other around like they were about to spawn for almost a year before the females actually dropped eggs, so it is possible they are just "practicing". Sometimes I swore it looked like my female fish were fatter during that time, but they never dropped any eggs until last month. I did not know that the anal vent sticking out meant they were ready to drop eggs, my female's vents always seem to stick out and I did not notice a change when they were spawning, but I wasn't looking that closely either. Did you read that somewhere? I am NOT doubting you, just curious as I don't remember hearing that before.
Also, I am not 100% sure on this, but the term "spawning", I believe, actually means that the fish are dropping eggs and/or the males are spraying milt, what you are seeing right now *I think* is just chasing and breeding behavior. I think that is how it got confused further up the thread that people thought your fish DID drop eggs already. If someone else confirms this, you may just want to say chasing/breeding behavior rather than spawning until the fish actually drop eggs, just so some people don't get confused....lol, like me!
When my females dropped eggs it wasn't what I expected at all, I thought they would shoot out in a huge bunch all at once, like I have seen salmon do. But what I saw happen was just a few coming out at a time, and I actually only saw them dropping once or twice, mostly it was just that they were all over the bottom of the tank and stuck to the filter and stuff. The eggs were also much smaller than I thought, just little white/clearish things about the size of a round pin head. I never saw the males spray their milt, but they obviously did since I got fry. Usually when fish spawn they do it very early in the morning too, so I was just lucky I saw it the one time, they have spawned 2 more times since and I missed the whole thing because I did not get up until 6:30 am.
Are you ready for babies? Do you have a breeding mop/mat in the tank or a separate tank to move the adults into once they drop eggs? The other thing I did not know is that the eggs stick to EVERYTHING and you cannot get them off. I always thought I would just take a few out (my fish dropped 1000's of eggs) and put them in a smaller tank, but because they stick to everything you cannot do that, so you either need a breeding mop or mat on the bottom for them to stick to, then you can move those into another tank, or you need a separate tank to put the adults into after the eggs are dropped-and that tank needs to be something they can stay in a long time because you cannot move the fry until they are at least 6 weeks old. And fry are TONS of work, they are little poo machines, lol! Sorry if you already knew all of that, just sharing some of the mistakes I made.
So just be patient, and keep watching them chase. Sometimes that can go on for ages! Just make sure the female does not get too tired or stressed. Doinga large water change can help too, so tonight if they have not dropped eggs yet, try changing like 50% of the water, and maybe make it a teeny bit warmer in the process. I am not a breeding expert, not by a long shot, I am just going off of my experiences. Good luck!