Alright, well you didn't say your tank was only 9" deep, (typo).
Honestly, one of the larger less lighting needy clams will quickly outgrow a 20 gallon, and quickly soak up nutrients such as calcium.
If you really want to try it, get a 150 watt HQI, look into either a viper k2 clamp lamp, by JBJ. Or a pendant, such as the outerorbit with luner lights.
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