Tallulah,
As Ken says you can use a Marine Actinic tube to make the light bluer, but this will reduce the amount of white light. With only 2 x 18w you may find it is not as bright & blue as you want (going by what you said in your e-mail). There are various other options depending on how much you want to spend - as I am in the UK I can give you an idea of options and prices over here. If you are in the US or Canada you will have a much greater range of T5 and Power Compact lighting available to you than we do over here.
An Interpet 24" Blue Moon tube could be used to replace the second tube in there for about £7.50. With the daylight you should get a reasonable intensity with a good blue hue.
If this seems too dark you might need to consider replacing the standard daylight with the Interpet Triton Triphosphor tube which is apparently noticably brighter that a standard tube - these are also about £7.50
Hagen's 24" tubes are 20w rather than 18w which will improve the lighting BUT you will need to be certain that that your fluorescent controller will handle the wattage. A Power-Glo 24" 20W will cost about £8.60 - this is described as "High Intensity Photosynthetic Deep Marine Spectrum". You could replace the daylight or the second tube with one of these. Hagen also do a Marine Glo which will probably be in the same price bracket.
Next one up is a Coralife 20,000k High Intestity Daylight - described as "high-intensity purified super daylight lamp". Prices for a 24" 20w in the region of £18.50
If you want a really bright & blue tank then you need to consider the T5 or T5 compact light fittings that are becoming more readily available. T5 lighting produces a much more intense light than the nearest T8 tube (standard 1" tube) which allows it to penetrate a greater depth of water.
This is where the prices start to go up.....
To retrofit your hood with Arcadia T5 lighting would cost about £56 for an electronic controller, £15 for a 22" 24w Marine White tube and £17 for a 22" 24w Marine Blue tube. You will probably also need 2 22" Arcadia T5 reflectors at £6 each. The difference in the lighting will be very noticable, the downside is a total cost of about £100.
An alternative is an 'overtank luminaire' - these are complete light units which stand on the sides of the tank and replace the hood. An ASL T5 Compact unit will cost about £68 which will take 2 x 55w tubes - the tubes are about £10 each and are available as Daylight, Actinic, 50/50 Daylight/Actinic, Red/Blue (for freshwater plants), Freshwater and Freshwater Colour. Total cost about £88 but you are now replacing 36w of lighting with 110w - you will see a massive difference. The only thing with this option is that it might be TOO bright for your fish - I don't really know what parrots prefer. The other thing you will need to think about is condensation trays to prevent too much loss from evaporation - these are only a few quid each but you would need to find the right sizes to fit your tank.
HTH, Eddie
P.S. All of the prices I gave above are from
Aquatics Online who I can highly recommend for dry goods. They don't always have the cheapest price on every item but in general I have found them very hard to beat.
thanks so much for that! your tank is gorgeous - its exactly the look i want. 
