Tune the EUR/BTC exchange rate according to the most important markets

The EUR/BTC exchange rate on Bitwala is constantly lower than the most important markets, e.g., at the time of writing, 1 BTC is valued € 3123 on Bitwala, while it is € 3164 on Bitstamp and € 3234 on Kraken. That is considered an additional tax by users as we buy BTC on markets but whe use them on Bitwala, with a constant loss in the change (Bitwala/Kraken gap represents a loss of 3.5%).