you would be able to keep almost anything under them in a shallow tank like you described. You would need 2 for sure to get coverage. But for anything over 16" in length i would recommend jumping up to a D120 LED as you will get much more power and will be able to cover the area with only 1 fixture and it would allow for dimming of each channel individually (blue and white). But if you are wanting to go with the PARs i would try 2 and see how it looks (i hung 2 above my 24" clown tank and it covered nicely).
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Nick
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