Just make sure you are not removing all the nitrate when doing the water changes to remove the phosphate, keep it in mind anyway.
Just add a small level teaspoon of bicarb, you dont need much, but you need to be consistant in the dosing, add exactly the same amount each time when you do the water change and this way you dont need to test the KH, because it should be the same. I remove exactly the same amount of water each week and add a specific amount of bicarb, i probably test about every month or so for KH but its nearly allways the same.
I would probably only add the bicarb if you are going to do 50% water changes each week, you probably could get away without it, for at least 6 months when i ran DIY CO2 i did not add bicarb or buffer my water and it had no bad effects, i did not lose any fish during this period, it was only when i started pressurised CO2 that i started buffering the water, but consistantly each week i do a 50% water change and reset the tank.
Just keep in mind if you buy a new tube locally that they may be the old dodgy type that caused all the overheating problems. They sell them in Aquatic Village in Brittas.
Are you dosing potassium sulphate btw?