Hi to reduce your diatom in your tank, you need to starve them.
Employ phosphate and silicate remover such as Rowaphos will help you win 50% of the battle.
Next is to invest in a good and efficient skimmer to remove organic before it builds up to a level that can feed them, a good skimmer will also help in the reduction of nitrate level which in turn translate into a healthier tank
Using RO/DI water/distilled might be cheaper in the long run, a nano DI unit from madpetz does not require a lot of space and is capable of producing 0 TDS water which is gd enough for any reef tank anytime. Not to mention there is no need to lug water container everywhere anymore.
If you find that you have already done all the above, activated carbon such as ROWAcarbon may help you further by absorbing the rest of the organic.
Last but not least, ask yourself a few questions: What kinda of food you feed to your fishes (frozen mysis is known to contain high level of phoshate), did you used a reliable test kits? Do all your fishes finished up all the food you feed?
P/S Water change using tap water will fuel your problem to a bigger pproblem
just some sharing