If you want to spend a few Euros then you might try Canna Terra, which is designed for use in potting soil. Canna Terra Vega (1L) | Grow Lights, Hydroponics, Grow Tents - Bloom & Grow
Or you can get a cheaper fert at your local grow shop. You can get a chemical fert that is cheap at a hardware store. The plant is turning yellow because it's needs a micro nutrient, like boron, sulphur, etc. So you want to pick a fert that has all that stuff in it. I like to use Dyna-Gro 7-9-5 and it would be great if you could find it in your area but I don't know if it's sold in Ireland. Here's what it's like: Dyna-Gro click on "products" then "complete plant nutrients." You want to find a fertilizer that is like this if you can because it provides everything plants need to grow. Alot of growers just walk into a shop and buy whatever looks good. That's how you get into trouble. Just follow the directions and don't over-do it.