You should set the PrimaryPart to one fish and weld them two. Anchor only the PrimaryPart.
From now on , all you need to use is CFrame, as colbert’s tutorial said.
Edit (06/30/2021): Due to the release of the new pivot APIs and the thankful deprecation of SetPrimaryPartCFrame, the legacy method of CFraming is now also appropriate and you do not necessarily need to use this welding method. Do note that the pivot CFrame is different from the PrimaryPart CFrame though, so you will have to account for that when writing your proxy objects.
Edit (05/31/2023): Roblox has an official code sample for model tweening! It’s very lightweight and covers the necessities…