Their Black Loch Fishery in Limerigg, south of Falkirk, which provides both bank and boat fly fishing, opened earlier this year.
Fraser Adams said: "We have been fishing together ever since we were youngsters and it was a natural decision for us to turn this hobby into a business."
The brothers also run their own separate businesses and have long banked with RBS.
The funding enabled them to lease a large lake, which is now stocked with a range of trout, build a club house to replace a derelict one that was already there and buy seven acres of land around the water.
They have also acquired 15 fishing boats and have put in an order for another five to expand their activities.
Mr Adams said: "We will be starting our ladies' competition fishing imminently and the addition of five new boats later this year means we can launch our men's competition fishing events next March."
RBS relationship manager Martin Blair said: "Fraser and Scott have brought a great source of revenue to the local community.
"They have achieved a great deal in recent months and I look forward to seeing the business grow its reputation and become a significant addition to the local community."
The loan to Black Loch Fishery came from financing that RBS sourced through the Funding for Lending (FLS) scheme, through which the Bank of England seeks to kick-start the economy by providing cheap financing to banks.
So far the part-nationalised lender has drawn down £750 million through the scheme, although it has allocated £4.3 billion of loans, ahead of future drawdowns.
In Falkirk, where the Adams brothers have established their business, RBS has issued £14m of loans through FLS.
RBS estimates these have saved customers in the area nearly £378,000 in reduced interest bills.