I personally wouldn't bother feeding them at all. Feed one and a half times what you usually would for 1 week before you go away (fatten them up a bit). On the day you leave, do a 50% water change and make sure all equipment is working correctly and that the filter media is clean in order that it won't clog whilst you are away and stop the filter working (don't change the media or you'll kill your fish, just clean it in tank water if necessary, not under the tap). Feed well and off you go on holiday.
The fish will be fine for 2 weeks with no food. Automatic feeders are not reliable and feeding blocks will just pollute your tank. Seems cruel, but really they'll be fine.
One thing, when you come back, don't just go straight back to feeding as usual. Feed 1/4 of usual amount for a few days, then raise it to 1/2 normal amount for a few days, keep increasing it gradually until feeding normally again.
This will ensure that you don't suffer any ammonia spikes as the bacteria will have partially died off whilst you have been away due to reduced feeding and the filter won't be used to the level of waste. Gradually re-introducing feeding upon your return gives the filter bacteria a chance to catch up.
