For coco, it's pretty important to keep it all flowing through and not let anything build up.
There are several ways people do this.
1: Make sure every watering has no less than 20% run-off. This makes the new feed a flush at the same time pushing out all the old salts and replacing them for the new ones (which are in the right ratio etc, not just left overs)
2: Incorporate a monthly flush of plain water to clear the slate and start fresh.
3: Some people alternate between feed and water. First time it dries out, you fertilize and then the next time you do water and so forth. If you were doing this one, it wouldn't be essential to have so much run off or regular full flushes. Some people get this one to go really well but personally I don't trust it since you could wind up deficient on some elements (my theory) before the next feed happens.
About your tap water's EC adding on top of your fertilizer... hehe duh. Water isn't fertilizer... so try to get as blank a slate as possible with your water so all the EC is useful. If you have to use water with an EC, yes you will add your fetilizer at the strength it says on the bottle and your end EC will be higher than that (because it adds on top of the base EC the water starts at). That's why you should know what your water EC is to being with so when you check it after you add the fertilizer you know how much is actually the fert or not.