Another problematic nutrient is magnesium (Mg). Magnesium is absorbed by plants in form of magnesium cation Mg2+.
You can add magnesium to aquarium from tap water because there is usually enough of Mg. Especially if you have hard water. On the other hand, there is a possibility that your tap water doesn’t contain enough of magnesium because your water hardness is made from calcium ionts. That can be really tricky.
Magnesium deficiency symptoms are very similar to iron deficiency. Leaves are pale and plants are losing a color. Magnesium deficiency symptoms usually show first on older leaves. That’s how you can find out, that problem is in Mg, not in iron.
It’s a good idea to add some extra magnesium because you can not be etirely sure that your tap water contains enough of magnesium. You can supply it in form of Magnesium Sulfate Heptahydrate (MgSO4x7H2O). You should add 1/2 tsp weekly for 25 gallon tank.