A letter has not gotten the retailers to pull them. It is the threat of Tunze/EcoTech, or whomever, canceling their wholesale agreement. In the end these web sites know that Tunze/EcoTech have a proven track record of being a good partner whereas Jeabo, Jebo, Odyssea, etc. leave disappointed, or even mad, customers in their wake over time. And what could the margians be 20 bucks per and no ancillary products to sell - at least Tunze has a full line that is worthwhile? If you ran a business, would you risk losing Tunze or EcoTech for Jebo?
IP Law is a long and costly process that I doubt any Aquarium Company in the US could afford to pursue. Even in Texas, where IP cases are easier, the road is not easy. This is where even if the claim is 100% accurate, then the law cannot help you unless you have significant means.
Every threat is empty until somebody takes it far enough. Judges don't send cease and desist letters (nastygrams) - the make orders that you have to follow if you are in the USA. I think that everybody just laughs at nastygrams anymore. If a judge found that these things did violate patents, then it would be illegal to sell them. Until then, it is a battle of who has more leverage.