Ruth, What about krill oil capsules? (I bought them because the bottle says that krill fishing is better for the environment. But when I got home and researched it, it's not. But I had them, and I've used them, and they seem to be more effective.)
Be sure to note the NSAID warning. But Z is on rimadyl, and I haven't had issues. But note the warning.
Fish oil has a higher percentage of EPA and DHA fatty acids, however krill oil has the advantage of having astaxanthin, an important antioxidant, vitamins A and D, and possibly other nutrients such as small amounts of phospholipids. Therefore, one option is to use a softgel or two of each, or alternate their use. Since phospholipids can be easily obtained from food, such as eggs, or lecithin ( phosphatidylcholine is found in lecithin ) one wonders if krill oil supplements are necessary when a person eats a lot of fish, takes fish oil supplements, consumes a variety of vegetables and fruits, and has a diet with adequate intake of phospholipids. http://www.raysahelian.com/krilloil.html
I alternate the krill capsules with fish oil capsules on Z. It seems to make a difference.
I don't like bottled fish oil. I used the bottled stuff all the time and have completely gotten away from it. I think it goes rancid faster than we realize (are you storing it in the refrigerator or a dark cool closet?). Or, at least, it oxidizes. And have you noticed how much oil you have to give of the bottled stuff for a dose vs. a capsule? I can't figure out why the dosing is so different.
To avoid having to give yet another pill to Z., I pinprick the capsule.
I keep the bottle of capsules in the darkest coolest cabinet I have (my refrigerator is packed with so many other dog things; I don't have room for that too.
The first onlynatural product you have listed has licorice in it. I stay away from licorice for my dogs because it's pretty potent and has some strong and potentially serious side effects. I'm sure you know them, but in case you haven't looked at them lately: http://www.personalhealthzone.com/licorice.html
With the other product, I am not familiar with most of those ingredients. I wouldn't feel comfortable commenting on it.
Are you doing anything for just general immune system boost? Vitamin C, Acidophilus, and a good multivitamin for starters? You were using milk thistle, right? Cranberry is a nice addition. Antioxidants and all that.