I'll make my suggestion based on the basic translation that are taught in the text book. 

*In my opinion, saying "futotta / futotteiru" instead of "debu" is better. Futotteiru inu and futotta inu mean "fat dog". 

Sakana wo tabeta inu wa debu desu. (The dog who ate the fish is fat) --> Perfect! 
(better: ...wa debu desu --> ...wa futotte imasu) 

Debu desu na inu wa/ga sakana wo tabemashita. (The dog who is fat ate the fish)

--> Futotteiru / futotta inu wa/ga sakana wo tabemashita. ("tabemashita" is the formal form of "tabeta".)

(The basic construction is "the dog ate the fish = inu wa sakana wo tabemashita", and "who is fat" is used like an adjective for "the dog", so it goes "futotteiru / futotta inu".) 

Debu na inu wa/ga sakana wo tabe mashita. (The fat dog ate the fish) 

--> Futotteiru / futotta inu wa/ga sakana wo tabemashita. 

*If these sentences are the answer for the question, for example, "which dog ate the fish?", we use the particle "ga". 

Good question. I hope you understand them but if not, please let me know.