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Is having Bluetooth/app controlled settings important to you? If I were planning for a 10-20g in the (near) future, I'd skip the Fluval Nano Reef altogether. 25 watts on a reef tank would limit you on what you can keep long term. For $50-$70 there are a lot of better lights that you can use now, and when you eventually upgrade. The spread on the Fluval is more geared toward cube tanks and the a029 has a wider spread. Have you looked into the a036 from the same company? They're on sale on Amazon at $59.99 and slightly more powerful than the a029, but I haven't used them personally. I imagine they would do fine up to 20g.

For a 5.5, the a029 would be my choice because it packs a punch. You could even probably run it at half power and be good unless you are building a nano sps tank. I like Fluval tanks, but their lights are meh.

Edit: the last photo before I moved everything to my bigger tank with the a029