pH crash

If you don't want to add any gravel to your set up then don't. A lot of people run aquariums with just sand or even no substrate at all. There are other ways to contol PH. Really, all you want is a stable PH that is consistent. The fish will appreciate a stable PH more than a certain PH for certain fish. They will adapt as long as it is not way out drastically of thier league as to what PH they can handle.

Marinemom

Yep, there are ways to increase the kH without a certain substrate or another. Use whatever you want to use. As you probably know, having a good kH (akalinity) value like 80 can help stabilize the pH and stop it dropping on you, but it also can tend to raise the pH somewhat. If the kH is too low then it can tend to make the pH more likely to fluctuate.

I was strongly urged to add gravel to my sand only substrate to avoid an iminent pH crash.

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first, bear in mind that many folks and many fish shops have not the first idea what they're talking about. there's no reason that gravel is better than sand if the sole purpose is to prevent ph decrease. if the gravel they were talking about was crushed coral, then the high carbonate level would help to buffer your water against ph decrease. there are coral sand mixes that will accomplish the same thing. if you have a reasonable kH, there's no reason for a pH 'crash' in the first place, and certainly not because you have sand or gravel as a substrate.

No differents from gravel or sand. One won't raise or drop your ph/kh unless it is crush coral. Find out how big your water "sponge" is kh. This will tell you how often you need to check ph and change water as NitrAtes and co2 will fill up the kh which could cause a ph crash. The bigger the number the larger the sponge and less often you have to change water:idea: