Ahti

Ahti means some sort of sea god in Finnish folklore.

Ahti is the God of the sea in "Kalevala", the national epic of Finland.

In the Finnish epic Kalevava, Lemminkainen is sometimes referred to as Ahti.

I think the name Ahti could come from the Finnish words ahdas (narrow), ahdistaa (oppress), ahdistella (harass)and jahti ->jahdata (chase, to chase). Because Ahti is the God of water, it is strongly connected to fish and fishing. In my opinion the name shows how Ahti easily "chases the fish in the corner", oppresses them or chases them in the narrow places which makes it hard for the fish to escape. Something like that, I guess. Or then it could be an ancient way to get fish.
The "d" changing to "t" is because of Finnish language´s extraordinary changing, many words bend just like that.

Not claiming to know anything about the origins of the name or to in any way to comment the idea of connecting it to words like "ahdistaa", I'd like to mention that if you mean consonant gradation (astevaihtelu), d never replaces t, it's always the other way around. (For example aita, "fence" -> aidan, "fence's".)

Ahti is also a term used in the Finnish Evangelical Lutheran Church. As a term, it is derived from Latin 'actus' and Swedish 'akt', which both mean act, part or chapter. In church it means a part of the history of the suffering of Jesus. I'm not sure if this explanation is complete -- My sources are rather limited.

Ahti is the name of the old God of water. People believed in Finland that Ahti rules kingdom under the water. His compliants are fishes and aquatic animals. He is anywhere where there is water.

The god of lakes, fish and water. He gives good luck to fishermen.