The seafood group will launch in Exeter on 18 March, with a 155-cover, purpose-built site at Exeter Quay.
Additional sites are set to open in Poole and Weymouth, Dorset later this year using the funding from HSBC - which did not disclose the figure it had provided.
Rockfish, which is chaired by Hawksmoor co-founder Will Beckett, has opened five coastal sites since its inception in 2010 and is planning to grow to 16 restaurants by 2022.
Tonks said the latest funding from HSBC would help the group continue to "change the way people experience seafood in the UK".
In January the chef told BigHospitality’s sister site MCA he had received a £1.5m minority investment from Gresham House Ventures to support Rockfish's growth.
Tonks also owns The Seahorse restaurant in Dartmouth, which was named the 40th best restaurant in the UK in Restaurant magazine’s annual top 100 list in 2018.