Eddyjack rose food is no good for an ericaceous plant. It contains the nutrients a rose needs not the nutrients a rhode needs. The two plants need completely different things.
Plants vary in their pH tolerance and the foliage of so-called acid lovers, such as rhododendrons which prefer a pH of 4-5, can turn yellow if the soil pH is too high. This happens because the alkaline conditions reduce the solubility of iron and manganese, so the plants cannot absorb enough of these elements to remain healthy. You can solve this problem by applying chelated fertilisers that contain iron, copper, manganese and zinc in a sequestered form that remains soluble at higher pHs but, as I said, if your rhode has been happy until now you must have a soil that is on the acidic side. All you need to do is stop using tapwater that has chalk in it, use rainwater if possible and add a little vinegar to green the leaves back up. You do not need to feed an acid plant if your soil is acidic.
Knowing without doing is like plowing without sowing."