What you are looking at here is an interesting ratio. First part is the ETHUSD/ETHEUR ratio, as given by Kraken. This simply gives the EURUSD ratio in terms of ETH. Theoretically, this should be the exact EURUSD as given by forex, but it sometimes isn't. So if we look at the (ETHUSD/ETHEUR) ratio and compare it to the real EURUSD ratio, it looks like this:
ETHUSD/ETHEUR/EURUSD. When it is perfectly 1.00 there is no arbitrage opportunities, but when including commission and slippage it seems to only be profitable above 1.01 and below 0.99. In practice this would be executed in an API being triggered to move EUR to USD through ETH when the ratio is at or above 1.02 and then back to EUR through ETH when the ratio is at or below 0.98. This would roughly give you a 3% profit in your EUR holdings when this is triggered. Of course this can be done between any 3 currency pairs. BTCUSD/BTCEUR/EURUSD, BTCXRP/BTCETH/ETHXRP, or anything of the form ab/ac/cb.
ETHUSD/ETHEUR/EURUSD. When it is perfectly 1.00 there is no arbitrage opportunities, but when including commission and slippage it seems to only be profitable above 1.01 and below 0.99. In practice this would be executed in an API being triggered to move EUR to USD through ETH when the ratio is at or above 1.02 and then back to EUR through ETH when the ratio is at or below 0.98. This would roughly give you a 3% profit in your EUR holdings when this is triggered. Of course this can be done between any 3 currency pairs. BTCUSD/BTCEUR/EURUSD, BTCXRP/BTCETH/ETHXRP, or anything of the form ab/ac/cb.