If the tank has been runnning that long and the problem with deaths is recent, I doubt it is phosphates (unless council has added something to the water supply). Our water is naturally high in phosphates too if I remember rightly and my fish are unaffected.
Did the fish all die within days of each other? 25L is not very big and you had 12 bottom dwellers in it, albeit small ones.
What is your usual routine maintenance (how often and how much water do you change regularly?) and have you tested the pH, ammonia, nitrite and/or nitrates? These would be more likely to cause deaths than phosphate levels I should think.