Red, brownish tip Elodea...

My Elodea develop some brown, redish tips.
My guess is:
1— low phosphates
2— low nitrates
3— low iron/traces cause by to muc magnesium

ph=6.9
kh=20 mg/l
gh=180 mg/l (to high)
Ca=40 mg/l
Mg= 19.5 mg/l (to high)
NO3= 3 mg/l
Po4= .25 mg/l
K= (dosed regulary)
Fe= .4 mg/l (uptake is maybe compromised by high magnesium)

This is happen to your Elodea???
How you fix it??

PS: I think a permanent thread should be post to present Plants that are sensible to lack or surplus of nutrient. That can be a great tool for the hobby...