Anything to give bacteria surface area to colonize on will help build it up. Personally I think lava rocks are ugly so i wouldn't put them into a stream but if function over form suits you go aheadI wonder something about streams ,would some lava rock mixed in a stream help build up the good bacteria build up .Also I know to keep my bird bath clean I add a small piece of copper plumbing fitting to keep water clean and yet they say copper is bad for fish is this true .Never tried it in my pond .My water lines in my house are not copper either .
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