Passive investors will see their costs fall even further, as fund manager Fidelity announced it is slashing the charges on its index tracker investment fund range.From 1 October, investors buying the Fidelity UK trackers through the firm's own Personal Investor platform will pay just 0.06% in fund fees.The US tracker will cost just 0.07%, Japan 0.1%, Europe (excluding UK) 0.1%, pacific ex Japan 0.13% and emerging markets 0.21%. The global tracker, which includes the UK, will cost 0.13%.Fees for Fidelity's trackers will also fall for investors on other platforms, though they will remain around 0.01%-0.02% higher than through Personal Investor.Investment charges have a major bearing on long-term portfolio performance, and low cost tracker funds have been growing in popularity. The Investment Association estimates a record amount of passive investments were sold to retail investors in 2014, rising 44% to £4.9 billion.The prices cuts are the second from Fidelity in little over a year, after previously reducing investment fees in May 2014.John Clougherty, head of UK retail at Fidelity Worldwide Investment, said: "There are still too many investors sitting in index funds with high charges, it has been pleasing to see that following our move last year, we have seen the cost of index funds start to come down. With our own range growing in scale, we are able to offer further price reductions for the benefit of UK investors."However, investors should be mindful that a firm's own fee for its funds is just one of many they face. If buying via a fund platform, they will also have to pay the platform's fees too - and these can vary considerably depending on how big an investor's portfolio is.Fans of low-cost passive investments may be interested to learn the world's second largest ETF provider Vanguard is looking to launch its own platform for UK investors, according to reports in IFA trade magazine Money Marketing today.