Help for live plant as a "secondary concern"

Hi all, studied a good amount, still have a question:



BACKGROUND
I'm planning a 2 fancy goldfish tank 60x45x45 cm (about 24x18x18 inches) of 121,5 liters (about 32 gallons). The aquascape will be an hardscape with round rocks. It will be lit only by natural light (no direct exposition to sunlight).
I'm inclined to put one or two little spots of live plants.
I do know that goldfish can eat or move them around and that's not a problem. In fact I may intentionally choose plants that taste good for them. I don't mind if I have to put new plants in the tank now and then, or if I have to trim them or remove them from the tank.
As a mere before hand exemplification I was thinking about Anacharis (favorite at the moment), Water Sprite (can require constant trimming), Vallisneria Spiralis (I read it can -slowly- grow whithout a good lighting).
Final statement is that I'll choose low-light, no CO2, "easy" plants, and I don't care too much for them to be "successful".


QUESTION
While I don't mind to if I have to put new plants in the tank now and then, or if I have to trim them or remove them from the tank, what I do care about are the possible negative effect of those plants... What if I don't guess the right time to remove them (if the plants, for example, are not healty)? What do I have to beware of?

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Thanks! I begin to see the whole picture
- If we will see that we're just "feeding" plants treats to goldies, we'll stop or consider a tank dedicated to live plants to feed to goodies;[/FONT]
- If we see a plant rotting for too little light, we will remove it from the aquarium and don't put another in without a good lighting system.
(and thank for the specific symptoms too look for)