I'm imagining your stand as a an 'II' shaped one. that is, 1 large top, 2 uprights, and 1 large bottom peice. Correct? you said that the water pooled on the large bottom peice. If it pooled on the large upper peice, and the affected area was say, the diameter of say, a CD, then I'd say forget it. If it's less, I'd probably try re-enforceing it with a brace that went at least 1.5x the length of the affected area, off to each side. So, if your area was say, 6cm, then I'd cut a 24cm peice (9cm on either side of the bad spot), screw it in from the top down (at least 4 screws in each side) through the particle board, then through the bracing section. Then, I'd cut a new upright, to go from the large base all the way up to the middle of the underside of the peice I just screwed in.
If it is the base, and it's, say, 12cm from either upright, then I'd just cut a peice and screw it down over the area. Agian, 1.5x the length of the bad spot, to either side. If it's near the uprights, you're probably gonna need to either get a new base, or a new stand altogether. In time, the uprights may punch though the weakened base that supports them, resulting in either a cracked tank or a tank on the floor. It's also possible, that since the ends of the uprights are just unfinished particle board mated to the base, that they themselves sucked up some water. If that's the case, I'd just go get a new stand.
If it is the base, and it's, say, 12cm from either upright, then I'd just cut a peice and screw it down over the area. Agian, 1.5x the length of the bad spot, to either side. If it's near the uprights, you're probably gonna need to either get a new base, or a new stand altogether. In time, the uprights may punch though the weakened base that supports them, resulting in either a cracked tank or a tank on the floor. It's also possible, that since the ends of the uprights are just unfinished particle board mated to the base, that they themselves sucked up some water. If that's the case, I'd just go get a new stand.