Ok, Sorry to dispute. But 2 watts per a gallon is NOT NEEDED. It really depends on the plants and what kind of work you really want into this. I have both Anacaris and Hornwort and they are in a tank that runs right at 80 degrees in temp as well. No issues at all with either plant because of temp. Ive also had these plants grow in as little light as 15w T8 over a 20 gallon High tank, along with Anubias, Java fern, Amazon Swords, crypts, Wisteria and about 4 other plants that are low to mid light. Would I recommend a light upgrade? Sure.. IF you want to do the additional things that can possibly go with it. Light is what drives plants, To much and plants suffer without Ferts and Co2. To little and not enough for some species to grow or even live. You could either do a DIY CFL light set up with 2 bulbs and have closer to 1w per gallon. Still low light but lots will grow fine with that. Or you could buy a T5NO 2 bulb light set up from Aqueon/Coral life. The bulb range you need is around 6,500 to 6,700K and as high as 10,000k. This would give the best spectrum for plant growth. I woul NOT do T5HO lights. This would be to much light and would most likely create an algae farm as well as drive the plants to need much more. Again it really depends on what YOU want to invest and do. High light tanks need Ferts and Co2. They also need specific plants meant for high light. if your wanting a good tank that grows plants like Hornwort then Id recommend shooting for not much more or around 1w per gallon or so.
Here is the info for Hornwort.
Information
[TABLE="align: center"]
[TR]
[TD="width: 240, align: left"]
Common Name: [/TD]
[TD="width: 240, align: left"]Tropical Hornwort [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Proper Name: [/TD]
[TD="width: 240, align: left"]Ceratophyllum submersum [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Category: [/TD]
[TD="width: 240, align: left"]Stem Plants[/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Temperature: [/TD]
[TD="width: 240, align: left"]10 - 30 C [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
PH: [/TD]
[TD="width: 240, align: left"]5.0 - 8.0 [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Lighting: [/TD]
[TD="width: 240, align: left"]Low[/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Growth Rate: [/TD]
[TD="width: 240, align: left"]Very Fast [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Difficulty: [/TD]
[TD="width: 240, align: left"]Very Easy [/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Origin: [/TD]
[TD="width: 240, align: left"]Africa, Asia and the America's[/TD]
[/TR]
[TR]
[TD="width: 240, align: left"]
Position in Aquascape: [/TD]
[TD="width: 240, align: left"]Floating [/TD]
[/TR]
[/TABLE]
Temp it can handle would be as high as 90 degrees. So I dont think its heat.
Hornwort can melt/shed when moved or replanted. Sometimes you have to just let it see if it will come back.
Again. Figure out if you want to put alot of work into a tank or little. Then you can decide on how much light you need and the plants you want. As well as what other things and expenses you will need to. Feel free to give a shout if I can help.