Potassium Nitrate - 40g to 500ml of water and adding 10ml per 100L of water would give you 5ppm NO3.
Potassium Phosphate - 15g to 500ml of water and adding 5ml per 100L of water would give you 1ppm PO4.
Potassium Sulphate - 55g to 500ml of water and adding 10ml per 100L of water would give you 5ppm K.
Magnesium Sulphate - 70g to 500ml of water and add 50ml once a week per 100L of water - this would give you 7ppm Mg.
Tank volumes are in US gallons.
To convert to UK gallons multiply these figures by 0.83 Your tank is 125L=33 us galls.
20-40 Gallons (76-152 litres)
20ml solution or 1/4 tsp KNO3 3x a week
12ml solution or 1/16 tsp KH2PO4 3x a week
5ml solution or 1/16 tsp K2SO4 3x a week5ml or 1/16 tsp traces 3x a week
Sunday ....... 50% water change. Add KNO3, KH2PO4, K2SO4
Monday....... Add traces
Tuesday...... Add KNO3, KH2PO4, K2SO4
Wednesday...... Add traces
Thursday...... Add KNO3, KH2PO4, K2SO4
Friday...... Add traces
Saturday...... Rest day
You will need the ferts which I link you to from AE. KNO3,KH2P04,K2SO4 and magnesium Sulphate 4 x 500ml bottles.
Mix ferts as per above and dose as above
The one problem people tend to have is how to check how much c02 is in the tank. This can be done one of two ways.