Can't figure out why my plants are dying???
Nicely explained, but hope he reads!Moppy, is this the first time you've had plants? It could be that the plants you have in the tank need more -- or less -- light than you're providing. Many standard tank hoods are good for low light plants only, and many of the plants that you see in the local fish store are intended for people who have more powerful lights and add fertilizers and CO2.
Another possibility is that you have planted them into plain gravel or sand, and they might need something with more nutrients. You might be able to add fertilizer tabs in that case, but I've never tried them.
Alternatively, have you added a medication to your tank? Some medications will kill some plants. Even a mild medication like copper can kill a sensitive plant like hornwort.
My last suggestion is that you tell us what kinds of plants you have so we can help you a little more. Some plants don't like to have their roots buried in the gravel, but like to be tied to a piece of driftwood or a rock. These plants, anubias for example, will rot if their roots are buried.
If you give us a little more information, we might be able to help! Good luck.