"Dag" or fish equals seven in gematria, as in the seventh day of the week which is Shabbos. It is brought down that fish is an integral food to eat by each of the three Seudas Shabbos. The Ohr HaYashar says that you should make sure to eat fish especially at Seuda Shlishis. Different reasons are brought down in the Heichal HaShabbos.
The Kotzker Rebbe says that since fish are the only living creature that we eat without any mitzvos accompanying it, such as Shechita and Melicha, therefore we eat it on Shabbos so that it too can be eaten with Mitzva. He also says that just like fish live in the water and do not see the world, so to we live in a world from where we cannot see the real world. On Shabbos which is Mei'ein Olam Haba we get a glimpse of the real world. Therefore we bring fish to our table so that they too can get a view of the world.
The Tiferes HaShabbos says that we can learn from fish the Middah of Bitachon. Fish eat other fish. When you find a fish inside the stomach of another fish you would expect to find the swallowed fish inside its predator, facing the same direction. As it ran away from the bigger fish and was finally caught, its tail would have gone in first. Yet the opposite is true, with the head of the smaller fish facing the rear of the bigger fish. How does this happen? The Tiferes HaShabbos explains that as the fish is chasing his catch another potential catch, swimming past in the other direction, gets swallowed in his mouth without him doing anything. This is the fish that Hashem intended to be his food.
From here we learn that we can chase in hot pursuit after parnassa that is intended for our friend, but at the end our efforts are in vain. We will not catch it. The parnassa that Hashem has allocated us will end up in our pocket automatically without any effort and without all our plans and schemes.
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