Rus seems like a good response. Just want to say, I don't know much about Miracle Grow, I just use cheap topsoil and they work just fine. Don't want to get all those fertilizer mix in the water. Your tank should definitely support 4 little fishes. I have 10 gallons and I have 5 adult fishes, 6 guppy juvies and countless MTS snails in there I suspect the guppies pop. will grow if I don't get it under control. Added I don't have any filter/airstone running for 2 weeks now. I've never see them gasping for air or anything so I would be concerned why that is happening with filters and only 4 fishes. The yellowness may leave after a few water changes. Have you used charcoal in your filter? It should help and it's also good to use it the first time you setup a tank as well.