I have not had any issues adding Prime directly to tank while filling 10g-120g tanks. Water changes are typically 40-50%. I do water changes weekly to 10 days at most.
If I had more room, filling barrels would probably be the better choice....just not practical.
I'll echo this. I've had good success with the water changer hose (Lee's, Aqueon, custom, whatever) for both drain and fill, add prime during the process so it can mix, good to go.
I did have some shrimp dieoff in my RCS colony after a couple water changes recently but it's a really small tank (2.5g) and newly set up/filterless, so those factors are both suspect. I switched to aging water in a butter tub and adding the prime during the fill to help with the problem...so far so good.
