Rhombus took human form and seduced Miya Shimada and by all accounts, he believed that he and Miya conceived her daughter, Debbie. After Debbie’s birth, Rhombus revealed its/his sinister side and attacked Tsunami. Miya defeated him with the aid of the ancient Atlantean sorcerer, Atlan. Atlan stripped the gem from its host and banished it to space. Atlan and then also imbued Tsunami with new powers and renewed youth. Atlan used a similar serum to extend he and his brother Haumond’s lives for millennia. Tsunami would now be able to sense Rhombus’ presence and was better equipped to defeat him. Many years later, Atlan himself would resurface claiming to be Debbie’s father (making her Aquaman‘s half-sister).
Tsunami never did, in so many words, claim that Rhombus was Debbie’s father, but she was fairly clear that Perkins was not the father. Miya allowed both Neptune and Debbie to believe they were father and daughter.