the duckweed does use up quite a lot of ammonia, but there are other things that you also need to filter. do yourself a favor and splurge on a good filter for it. i understand building on a budget, believe me, but the one thing i absolutely NOT do on the cheap, is life support systems. splurge on a good canister filter like a zoomed 501 or something to that effect. as far as heating, i use good quality aquarium heaters. either eheim or fluval. they are adjustable and hold their settings well. they can be hidden under false bottoms easily and are almost bullet proof. just remember to leave access to remove it if it fails.
a t5ho build is the way to go if you have access to one. i use a 4 bulb unit to light my mantella tank and the plants grow like crazy. cfl's are also good and should be good for a shallow tank like a 20L. stick in that color temp range (6500k) and all should be good.
i am lazy, and get my fruit fly cultures from petco. they last about a month. i'm sure lynn can give more info on the Josh's cultures, since i believe that's where she gets her's from?? btw, i use melanogaster flies.





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