Duane gave some very good options. Having said that, IME CL's also generally do quite well in hard mineral rich tap water, so other fish can certainly be utilized as tank mates. Just not overly aggressive specimens or the clowns will have to fight for scraps at meal time, and will stay hidden much of the day. I learned this the hard way years ago. When allowed to live in a safe, peaceful environment, instead of hiding all day under a rock, or inside a cave, this is what you can experience. The photo below was taken in a clown loach only 125 gallon set up that I had running years back. There's only a few set ups that I miss from the past, this is definitely one of them.
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