The 18" 15W tubes you've just bought will require a 15W Starter unit each, or will need to be fed by a 15W Dual starter. The 15W starter will also run lower rated (W) tubes.
If you have higher rated tubes, they will need a higher rated starter, usually about £20.
If you run higher rated tubes in a lower rated starter, you risk the starter failing at best, and potentially a fire at worst
However, if you want greater light levels, You can pick up T5 starters for less than £40, with 16" long, 36W lamps for about £15.
Aquaessentials have these, with the T5 version of the daylight plus tube, which is apparently quite good for plants.