Originally Posted by smit3183
It's a 55g with just a standard fluorescent bulb. 3 platies, 6 cherry barbs and 5 zebra danios. Was told not to use the flourish until the plants established themselves which he said is a couple of weeks
I have read similar, though I do not follow this myself. When I set up a new tank I plant it well on day one, with fish added, and I dose Flourish Comp the day following and thence every week. This makes sense to me, as there will not yet be organics in the substrate to provide nutrients, so they have to come from somewhere.
It is true that plants when moved to a new environment can take time to adjust and establish. This can vary depending upon the plant species and the changes--water parameters being hardness and pH can be significant to the plant, plus different lighting. I'm not sure I've ever really thought of it at the time, but I think my plants generally show new growth (leaves) within a couple weeks. Again, different plants can react differently.
Existing leaves yellowing/browning can be due to just the environmental change. Or perhaps if something is missing, though 4 days is not sufficient time to really judge this.
I would look into your light though. A single T8 fluorescent tube full length (48-inch) over a 55g is fine, you are at low-moderate lighting, and a majority of plants will grow with this. But the tube itself may need replacing if it is the one that came with the fixture. If you look at one end, there will be printing; tell us what it says and we can advise further.