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why would you get rid of the t5, it has more output than a t8 does. If anything I would get rid of the t8 and add another t5, or get rid of them both and get a dual t5ho fixture. (probably the cheapest route)
Okay. Using WPG as a form of measurement with t5's is inaccurate, because they deliver more light intensity per watt than a t12/t8 does.
Go with the t5ho's and you'll have all the light you need.
Also, the kelvin rating doesn't compound, it's a measurement of color temperature, so 4 x 10000k lights would still be 10000k.
Here's a basic chart of kelvin rating color range. It's just a general chart, for aquarium lights you can get up to about 10000k before it starts becoming noticeably blue.