roses in themselves are not toxic, edible even, but they are usually heavily treated with chemicals to keep bugs from eating them. Roses are also going to get mold. Another note, small mini roses are toxic, plant and all. I suggest if you want flowers in your tank, buy sweetpea or African violet. Sweetpea grows taller, 6-8 inches

Also, orchid, but these need good air flow, the other 2 are easier