I tried cod liver oil( damm predictive text!) for general fishing but can't say it made any difference! I used the soluble capsules which were past their sell by date! I just added it to either GB or micros!
Used to be quite popular back in the late 90s early 2000s it was featured in anglers mail in 2003, it is a horse joint supplement super codlivine now about £15 for a 2.5 kg pack you use it at 10 % of your groundbait mix and it turns it red, if you google " codlivine for tench "the anglers mail article gives a few gb mixes using it, no idea how to do a link thingy, mainly for tench and bream. as above strong aniseed smell and lots of fish oils.