hummm interesting idea!
I wouldn`t have thought u`d need filteration since no ammonia is being produced. However i would think u`ll need to water change occasionally.
The only problem i can see you facing, is since you want the riccia to grow well (since it is a harvest tank) you`ll need strong light, but then u face the problem that if `ve only riccia in the tank with strong light, algea will grow.
Also if you add strong light, you`ll need to add CO2. Then after adding CO2 and strong light, you`ll need to fert ! Thats one hell of an effort just to grow riccia!!!
Phlawed, if i remember correctly, you made that slate with grid for your riccia to grow. You didnt have very much and i presume your trying to cultivate some more so you can fill it? I have riccia and can tell you that if your using CO2 and over 2wpg then your riccia will grow. Imo your best bet is to make a small slate, just like the one you did but one that will fit on all your riccia and completely cover it. Then just sit it out and let it grow, 3 months down the line it will burst open, harvest your new riccia and then cover your new slate!!! I think your only problem was your slate was too large. Maybe i have presumed too much and you dont want the extra riccia for this, but either way good luck, i seems a lot of effort to grow some riccia (onother option is to just float it at the top of the tank!)
Good luck
Chris