Deca is known to stimulate progesterone receptors (you know the feminizing sex steroid hormone) however, one of the weird things about Deca that gets over looked is it can also increase estrogen Levels Via a secondary aromatase reaction, prompting the development of gyno.
Deca can and does accelerated Testosterone and others androgens to aromatization. (Break down to estrogen faster than the normal stat).
HCG—does not aromatize, but can cause significant increase in test which does aromatize and it also stimulates aromatization in the leydig cells.
Clomiphene works by blocking estrogen at the pituitary. The pituitary sees less estrogen, and makes more LH(Luteinizing hormone). More LH means that the Leydig cells in the testis make more testosterone.
Nolvadex, is a agonist/antagonist, better way is to say it competitively binds with estrogen receptors in various targeted tissues, with Nolvadex it is very common in blocking estrogen in the chest area. (Many forums of breast cancer is because of estrogen, Nolvadex is used because Nolvadex blocks estrogen in these cells.) However, Nolvadex also increases production of FSH (Follicle-stimulation hormones—better yet increases your sperm production) , and LH.
So now that we have cleared that up. Let’s look at a few things, if you take HCG estrogen will be created in the testis, which an AI cannot block because it is not estrogen converted from Testosterone.
So you have two problems with your question. “ if you are running hcg clomid with your trt and add deca will it have any effect on fertility”
1. You have no AI to keep your estor in-line encase shit.
2. Your only using Clomid, which only blocks estro at the pituitary(you don't need clomid)
So in short if you were going to do this, you would need an AI on hand. And you would not run Clomid you would run Nolvadex because what it offers with Sperm production, and the ability to stop estrogen in the Chest/Nipple cells area.