Answer: Based on your description I am not certain what is happening. If the lower leaves are going first, it may be lack of light and/or overwatering which has caused root damage. This plant likes lots of indirect light when grown indoors. The soil should be allowed to dry a bit between waterings but not so much that the plant wilts.
If the tips of the stems are going first, it might be too much direct light or too little water or air that is too dry. (It is happiest in a warm and humid location.) It might also be due to some root damage related to a build up of fertilizer salts in the soil or to deteriorated potting mix if it is an old plant, or even very dry air.
It could also be that the plant needs to be cut back hard (almost to the soil) and/or repotted into fresh soil and maybe a larger pot, if you have had it for a long time. (Many gardeners cut this plant back very short when it looks too leggy to encourage vigorous new growth.) I have also found that this plant needs to be renewed after a year or two by rooting tip cuttings and starting a new one.
If it is an infection, you would see spots or dots or a pattern to the discoloration before it spreads over the whole leaf. This is really hard to diagnose long distance. But, better air circulation, correct watering, and not overfertilizing would all help with this. Avoid wetting the leaves when you water. Cutting it back very short or starting over with rooting new healthy tips would be a good way to try to correct it.
As far as pH, I have not found a specific ideal pH for this plant. In general, if your other houseplants are doing well then it should be adequate for this one as well -- it is not considered to be a finicky plant at all. (The acceptable range would be from about 5.5 to 7.0.) You could certainly use aquarium water to water your plants occasionally, though, when you are changing out the water for the fish.
I hope this helps!
Q&A Library Searching Tips