I would say from experience from many walstads,that air pumps do indeed force co2 out of the water column. What I can recommend that will work for your grass is a very very small water pump. U can buy these on Ebay for less than 5 quid. It's a shame I'm so obsessed with the cleanliness of my tank otherwise I would remove the grass and fill with Riccia. My two 30 percent water changes a week allow for the water to stay clean but going down this route has affects. I think it's important for people to know that it works and works well but can not imitate a fast flowing tank. I think adding a water pump would not change the fact it's walstad in any way. After all the plants are the best filters u can buy. The begging of the walstad cycle is the most painstaking as things have not yet developed and progressed. This has been a battle but keeping on top is fine. I think u will be fine if u follow the steps etc. avoid air pumps if u have fast growing pumps. Water flow and plants is enough to build oxygen for the fish please do let me know your plans as I think it would be great for other walstad users to start commenting. Although I have ordered filters for these tanks I have 2 other tanks that are walstad, one being the jungle tank that is working amazingly well. I change it's water now once a month and clarity and cleanliness is superb. So it does work. It absolutely does
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