Giardia in dog

Hi all. My dog seems predisposed to giardia or something. When he was a puppy he had it 3-4 times and it was difficult to get rid of. It was a lot of antibiotics. I took the advice of another vet and they said to feed him a digestive support food and let his body build a resistance and that seemed to work. I thought we finally nipped it in the bud but he's 3 now and he recently got it again about a month and a half ago. We treated but it came back. My regular vet readjusted the meds and put him on more and it worked but now two weeks after treatment ended it is back again. I really don't want to continue doing the antibiotics if they are clearly not working.

I am cleaning up poops. I was wiping his butt every time he pooped. I been hosing down any leftover residue since it is goopy and sticking to blades of grass. I didn't give him a bath this last time but he does swim regularly in a saltwater pool.

I looked into alternative treatments and added some drops into his food and increasing fiber. He is a frustratingly picky eater so it's kinda difficult to get him to eat anything.

Anyways, any suggestions appreciated.