I could not find a clear answer on google so plz tell me
Well, this is simple. Sharpedo and Carvanha can only be found on Route 22, so that is where you go. Sharpedo uses a Super Rod and Carvanha uses a Good Rod. I will edit if I can get more specific.
Edit: The most specific I can get is to fish near rocks or places with things blocking the bobber. This gives you a higher chance of reeling in pokémon so you can keep the chain. This page helps: https://m.bulbapedia.bulbagarden.net/wiki/Fishing