well 1st you need to decide on what dosing method best suits you. E.I, PMDD+P04 or all in one soloution.-

http://www.theplantedtank.co.uk/dosing-methods.htm
i would recomend EI, its simple, very effective, very flexable and ideal for a highlight tank which yours will become once you get the new lights.

E.I is a mixture of

__Potassium Nitrate__(KN03)

__Potassium Phosphate__(KH2PO4)

and

__Trace Elements__-this is a mixture of many elements e.g. Iron, Magnesium, Copper, calcium and Zince ect ect.

these are all easily avalabe here is one link where you can get them all-

http://www.aquaessentials.co.uk/index.php?main_page=index&cPath=414_145_146
there are other things that you can add depending on your water but i wont go in to that. you can read here-

http://www.theplantedtank.co.uk/EI.htm
so, once you have your 3 basic ingredients you need to decide if you want to add them straight to the tank as a powder (dry) or you can make a stock soloution.

i prefer to make a stock soloution-on the web site i have gave you the link to it shows that you need to make 2 soloutions out of the 3 ingredients.

Macro Solution

33g Potassium Nitrate

7.2g Potassium Phosphate

250ml Water

(All these values can be doubled if you go through it quite quick)

Trace Mix Solution

10g Chelated Trace Element Mix (7% Fe, 1.3% B, 2% Mn, 0.06% Mo, 0.4% Zn, 0.1% Cu)

250 ml Water

0.5 ml Normal Hydrochloric Acid

it also says that the hydrochloric acid is not vital, its just there to preserver it a bit. its also recomended to use RO water or deionised water. but many just use tap water.

so now you have your 2 bottles of soloutions ready to add it says

Using Solutions

Using the daily schedule below add the following 3 times a week:

5ml of Macro solution per 50 litres of water

2.5ml of Trace solution per 50 litres of water

here is my working for dosing E.I in your tank.

__MACRO__
120L/50L = 2.4 ---> 5ml x 2.4 =

__12ml__ 12ml is the ammount you add 3x a week.

__TRACE__
120L/50 = 2.4 ---> 2.5ml x 2.4 =

__6ml__ 6ml is the ammount you add 3x a week.

as the method hints this a estimate, we aim with this method to provide more than enough nutrients for the plants to survive and grow and stop any deficiency. dont worry if you add 1ml more to your tank one day and 1ml less the next.

this is just one route to dosing that i chose to explain, there are others on james website that you might find easier.

i hope this helped you. dont hesitate to ask any questions about it.