Just so everyone knows this is what the fish tank is. Not sure but I think it increases one of the stats of all fish weapons: laser fish, fish, heavenly sword e.c.t :
Laser Therapy | Soul Knight Wiki | Fandom
Found a bug where if you have your fishing rod out when you pick up a weapon, you can then carry infinite weapons : r/SoulKnight
Making a Laser Fish From Soul Knight IRL - YouTube