Warmer Then Your Average Alligator Weed Flea Beetle.
From what I've read about Alternanthera philoxeroides, 26ºC would be close to the lowest optimal temperature for Alternanthera of 25º C. Upper optimal temperatures for population growth are around 32ºC. My guess is your plant would be somewhere in that range.
It is a real pain to grow it submerged if under high/medium light. It is very tricky with CO2 too. Even Tom suggested shading it for a more consistent growth. It will grow but after some weeks/months, old leaves will decay too early. I'm curious to see the comments of someone having it for many months without issues
I think mine is a cardinalis. I have bought it about 1o years ago for a different tank. This one has a very different setting. 240 watt HQL light above a 90 gallon tank.
I struggle with green alg on the leaves. I have raised the CO2, NO3, PO4 and the trace elements.
So I hope it will does better now. I was not sure about the temp. For some reason I cant recall no I had change the temp from 78 to 76. I have it now on 77.
I know it grows a bit slower. In the old set up it did even slower. Now I think it will, grow a inch a week.
It was hard for me and mu wife to adjust to the EI way of thinking. But now I am convinced.
Your Temp is quite good. I think your minimum temprature should be more than 25 degree. ok
The striking green and crimson or lilac foliage and upright growth habit of alternanthera cardinalis and lilacina make them ideal freshwater aquarium plants. They do best in soft water with a slightly acidic pH, moderate to bright light and a temperature range of 75 to 80.6 degrees.
It did very well when I changed my flow. But then there was a other corner where the plants start to slowdown.
So I set it back. Then the Alternanthera get in to trouble again.
So I installed a other outflow thingy en increased the CO2 to 40 ppm
Every thing is bubbling great so I think I am on the right track.
The HGL lamps I use have two settings 80 or 120 watt.
The are both on 120 now. My idea is give it a go with 40 ppm CO2 and plenty of al the other stuff and looks what happens.
When that does not help, I set the lamp above the Alternanthera on 80 watt.
I want the alternanthera to be a big peace of the scape. Almost to the top.