Which species did you manage to grow and for how long?
How long have the bulbs used for so far?
Were the lights the same distance above the tank when you were growing plants as they are now?
How tall is your tank? Is it the 21" tall version?
By no means is a lack of light 100% the only answer that could explain the damage to your plants, but it does fit the plant symptoms very well and what you have mentioned in this thread on several levels so it is the most likely issue. Also, 48 watts of T5 HO light over a 36g tank is not very bright light. It should be in the medium light category if the reflectors on the fixture and the distribution of light is good. This also only applies if the tank is fairly short. For very tall tanks you need much more intense lighting to penetrate deeper into the water column. For comparison I am running 4x54 watts of T5 HO over my 40g tank (which admittedly may be a tad bit excessive).
It all turned brown and died in the first four to five days of being in the tank.
This time frame fits very well with how long it takes stem plants to die from a lack of light.
It was growing nicely on the top but still ultimately died.
This fits as well. Plants with a lack of light will tend to keep the upper leaves only and all the lower leaves will suddenly die and rot at once. Leaving a large amount of dead leaf tissue but an intact stem. Only upper leaves will be alive.
I have had similar troubles with ludwigia species in the past and I have no idea what to do with this big empty section in my tank.
Repeat problems with stem plants but not other low light species like the crypts in your photos matches low light conditions.
Tank: 36g bowfront
Light: dual bulb T5HO
CO2: GLA Primo pressurized system w/ inline reactor
Filter: Fluval 406
Photoperiod: 8 hours (4pm-12pm)
Fertilization: 3/week Macro Soln.(KNO3+KH2PO4) 3/week Micro Soln.(Plantex CSM+B) all according to EI
Temperature: 79° (heater won't go any lower for some reason)
As to your other questions: There were no diatoms. The plants were delivered overnight with a heat pack and all together. Pictures to follow!
Looking at the problem from another angle, we can eliminate certain possibilities.
A deficiency is not likely since the symptoms are so severe and sudden. A deficiency would take about twice as long and would not be nearly as severe. The symptoms also do not match with deficiencies.
The symptoms might match a toxicity of ammonia though you'd expect to see damage on all plant species including stems and crypts. Damage would be on new and old leaves as well since ammonia would be all over the water column and would damage new and old leaves alike. The same is true for most other toxins and algaecides.
Shipping damage might be an explanation for the damage, but why would that spare the new leaves and stems?
Furthermore, looking at your tank photos they look dim, note the shadow cast on the wall a few inches from the surface of the water. This is likely where the adequate lighting is to be found, not so much at the bottom of the tank near the substrate. This is usually a good signal that the tank has low/lower light.
Best to start at square one and make sure the lighting isn't the issue. More light won't hurt the plants. If you have other lights handy you should do a trial run and add them to the tank, or lower the current fixture so it rests directly on the surface of the tank. This will greatly increase the amount of light reaching the plants.
Another thing to keep in mind is that reflectors make a big difference. I once had a 90g tank and set up 2x250 watt metal halide lights above it thinking that 500 watts of metal halide light would be plenty and then some to grow whatever plants I wanted, but in fact due to the quality of the reflectors I was barely able to grow stem plants which also showed similar problems as your plants did. Once I switched to T5 HO lights with individual reflectors the issue stopped and I could grow HC right on the bottom of the substrate.
From:
http://deficiencyfinder.com/?page_id=757
Here is a light deficient stem plant (not my 90g tank) that has lost its lower leaves and then the last photo shows the same tank with a better light fixture a few weeks later.
Same tank later on with a better light fixture.