Hi Ecro,
I'm with BigC and Geo7x on this. Holiday blocks are generally a bad idea.
They have a tendancy to just rot in the tank and pollute the water massively. I have heard many stories of people feeding holiday blocks and returning to a tank of dead fish.
Fish will happily go without food for longer than you think. Anything up to 2 weeks should be absolutely fine, and it is certainly less risky than leaving a pollution block in your tank. Even if your neighbour can only come in once, half way through your holiday, that is what i'd recommend.
I am currently looking after my friend's fish whilst he is on holiday for 2 weeks. I didn't visit at all until 1 week into his absence, i then visited, changed 50% of the water and fed the fish lightly. No problems at all.
The best way to do it is to feed slightly more than your usual amount for 1 week before you leave (fatten them up a bit). On the day you go, change at least 50% of the water and check that all equipment is working correctly (most importantly that your filter is not blocked to ensure it won't stop working when you are away), feed the fish and off you go. If you do need to clean the filter, make sure you use old tank water, not water straight from the tap.
Ask your neighbour to come in every week or every 2 weeks and feed the fish lightly. You are always best to leave a set amount of food for your neighbour to use, as people with no experience of fishkeeping always overfeed fish (i don't know why?), and leave strict instructions that no extra food has to be added. Make each feeding whilst you are away around half your usual amount.
When you are away, because the fish aren't eating so much, they aren't producing so much ammonia and some of the filter bacteria will die back. Upon your return, it is important not to commence usual feeding straight away as the filter will not cope and you will suffer an ammonia spike.
Instead, start with feeding 1/4 of the usual amount for the first few days, then increase it to 1/2 for a few days, then 3/4 for a few days and then finally up to your usual amount. This will ensure that your filter bacteria have time to catch up again.
Ludwig, i understand that you have had success with feeding blocks and are giving advice based on your personal experience as we all do, but you have been lucky. Many people have had a very different experience with feeding blocks. They are unreliable and very risky. Not feeding the fish (for anything up to 2 weeks) is a much safer option.
I'm very curious why you advise not to change any water for 1 week before leaving. That doesn't make any sense to me at all?
Ecro, hope this helps you. Good luck!
BTT