Investment in the business and a squeeze on interest margins put a dent in the profit of Teachers Mutual Bank.TMB's profit fell by 15 per cent to A$27.1 million over the year to June 2019."Profit margins are tight of course, in this world of low interest rates. In this environment, margins a squeezed a little bit as you know," Steve James, TMB CEO said."There were some significant things we did. We put on 50 staff, funded a bigger tech spend and robotics, and we removed a few fees."One of these was the $5 monthly account keeping fee, dropped in February.The bank continues to grow well above system, with home loan growth of 9.8 per cent.Its balance sheet increased by 13.61 per cent (or nearly $1 billion) "on the back of strong deposits from households," James said. "Household deposits increased by $695 million last financial year and are now over $6 billion, the most of any Australian mutual bank."TMB is also sinking resources into its Health Professionals Bank brand.