Red plants for a low-tech... Hmm.I've used: (not my pictures)
Crypt wendtii 'bronze' and crypt wendtii 'red'
See Plant Profiles
Echinodorus var 'Red Flame'
Echinodorus Ozelot 'Green' or 'Red'
Ludwigea Repens/Ludwigea Natans (same plant)
May want to try Cabomba palaeformis
See plant profiles
Many Hygrophila species are red- will have to look them up individually. Hygrophila 'Sunset'
Also, Nymphaea species are sometimes quite red. Nymphaea pubescens is a nice one that takes well to low light.